Brain Advance Access originally published online on May 26, 2006
Brain 2006 129(7):1803-1821; doi:10.1093/brain/awl140
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Functional circuitry underlying visual neglect
1 Laboratory of Cerebral Dynamics, Plasticity and Rehabilitation Department of Anatomy and Neurobiology, Boston University School of Medicine Boston, MA 2 Centre for Brain and Mind, Departments of Physiology and Psychology, University of Western Ontario London, Ontario, Canada 3 School of Engineering and Science, International University Bremen Bremen, Germany
Correspondence to: R. Jarrett Rushmore, PhD, Laboratory of Cerebral Dynamics, Plasticity and Rehabilitation, Department of Anatomy and Neurobiology, Boston University School of Medicine, 700 Albany Street, W702, Boston, MA 02118, USA E-mail: rushmore{at}bu.edu
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Visuospatial neglect is a common neurological syndrome caused by unilateral brain damage to the posterior and inferior parietal cerebral cortex, and is characterized by an inability to respond or orient to stimuli presented in the contralesional hemifield. Neglect has been elicited in experimental models of the rat, cat and monkey, and is thought to result in part from a pathological state of inhibition exerted on the damaged hemisphere by the hyperexcited intact hemisphere. We sought to test this theory by assessing neural activity levels in multiple brain structures during neglect using 2-deoxyglucose (2DG) as a metabolic marker of neural activity. Neglect was induced in two ways: (i) by cooling deactivation of posterior parietal cortex or (ii) in conjunction with broader cortical blindness produced by unilateral lesion of all contiguous visual cortical areas spanning occipital, parietal and temporal regions. The direction and magnitude of changes in 2DG uptake were measured in cerebral cortex and midbrain structures. Finally, the 2DG uptake was assessed in a group of cats in which the lesion-induced neglect component of blindness was cancelled by cooling of either the contralateral posterior parietal cortex or the contralateral superior colliculus (SC). Overall, we found that (i) both lesion- and cooling-induced neglect are associated with decreases in 2DG uptake in specific ipsilateral cortical and midbrain regions; (ii) levels of 2DG uptake in the intermediate and deep layers of the SC contralateral to both cooling and lesion deactivations are increased; (iii) changes in 2DG uptake were not identified in the contralateral cortex; and (iv) reversal of the lesion-induced neglect component of blindness is associated with a reduction of contralesional 2DG uptake to normal or subnormal levels. These data are in accord with theories of neglect that include mutually suppressive mechanisms between the two hemispheres, and we show that these mechanisms operate at the level of the SC, but are not apparent at the level of cortex. These results suggest that the most effective therapies for visual neglect will be those that act to decrease neural activity in the intermediate layers of the SC contralateral to the brain damage.
Key Words: visuospatial neglect; superior colliculus; animal models; parietal cortex; visual perception
Abbreviations: 2DG, 2-deoxyglucose; MSs, middle suprasylvian sulcus; pMS, posterior middle suprasylvian; SC, superior colliculus; SGI, stratum griseum intermediale; SGP, stratum griseum profundum; SGS, stratum griseum superficiale; SO, stratum opticum
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Received March 7, 2006. Revised April 24, 2006. Accepted April 26, 2006.
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Introduction |
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Visual neglect is a neurological syndrome most commonly caused by unilateral brain damage to the posterior and inferior parietal cerebral cortex and is characterized by an inability to detect or orient to stimuli or objects presented in the contralesional visual hemifield (Heilman and Valenstein, 1993
; Mesulam, 1999; Driver and Vuilleumier, 2001; Kerkhoff, 2001; Mort et al., 2003).
Patients with neglect behave as if one half of visual space does not exist and typically deny the presence of a defect. However, neglect is usually not a sensorial problem of absent or impoverished visual signals from the neglected field because a variety of treatments such as the application of cold water to the ear canal ipsilateral to the neglected field (‘caloric stimulation’) or vibration of the dorsal neck musculature causes the patient to once again become aware of stimuli in the neglected hemifield (Pierce and Buxbaum, 2002; Schindler et al., 2002; Kerkhoff, 2003). Such treatments for neglect have only transitory or incomplete efficacy. These phenomena inform us that sufficient neural circuitry exists both for detection of stimuli in the neglected hemifield and for producing orienting responses to the stimuli, but that the circuits are prevented from acting. As a consequence, a potential cure may be effected if the secondary circuits were identified and reactivated through interventional therapy or training. However, precise dissection of circuits in humans with neglect has proven difficult owing to lesion variations and the emergence of neural and behavioural compensations following neglect-inducing damage to posterior parietal cortex. The detailed study of neglect requires a stable experimental model of the syndrome.
To that end, we have studied the cat model of neglect. Just as in humans, damage to the posterior and inferior portion of the cerebral cortex in the cat results in a syndrome with the same features as neglect (Hardy and Stein, 1988; Lomber and Payne, 1996; Payne et al., 1996a). Stable versions of neglect are induced either through unilateral cooling deactivation of posterior parietal cortex (Payne et al., 1996a) or in conjunction with the generation of a broader cortical hemianopia in which all contiguous visually responsive cortex is removed (Lomber et al., 2002). In the cat model, lesion and reversible deactivation studies have allowed for the generation of a hypothesis of neglect (Payne and Rushmore, 2004). This hypothesis suggests that neglect arises because of an imposed interhemispheric imbalance of the orienting system, which works to bias the system towards orienting to stimuli in the intact visual hemifield, and skews the system against stimuli presented in the neglected field.
The neglect-inducing lesion or deactivation of posterior–inferior parietal cortex has been hypothesized to cause an imbalance in activity between brain hemispheres through imposed disruption of two main circuits.
(i) Cortical circuits: The neglect-inducing injury or deactivation of posterior parietal cortex silences or denervates transcortical commissural projections to the homologous cerebral region. In a functional sense, the injury is thought to release the contralateral homologous parietal cortex from inhibition and result in an elevated level of activity—an excitability that then is translated along efferent projections to overactivate the cerebral and subcortical orienting circuitry in the intact hemisphere (Payne and Rushmore, 2004).
(ii) Corticocollicular circuits: The neglect-inducing manipulation of parietal cortex removes a major source of input to the superficial visual layers of the superior colliculus (SC; Harting et al., 1992), an action that feeds forward and decreases excitability in efferent collicular circuits. Most notably, efferent and inhibitory projections to the contralateral SC become deactivated, resulting in a disinhibition of the contralateral SC, a heightened state of activity that feeds back to the ipsilateral SC as a higher inhibitory tone (Kinsbourne, 1974, 1987, 1993; Hilgetag et al., 1999; Payne and Rushmore, 2004). The end result is that the ipsilateral SC is deactivated more than would be expected than on the basis of simple denervation of parietal inputs, and the contralateral SC becomes more active than normal.
Accordingly, our hypothesis is that the neglect-inducing lesion of parietal cortex induces interhemispheric asymmetry both in the cerebral cortex and in the SC. To address this hypothesis, we prepared three groups of cats. In one group (Group A), we induced neglect by unilateral cooling deactivation of parietal cortex. Cooling does not induce neural compensations, and a stable neglect state is generated for the duration of cooling (Lomber and Payne, 1996; Payne et al., 1996a). In another group (Group B), we made large unilateral visual cortex lesions. While small lesions of posterior parietal cortex produce only transient deficits in visual spatial attention (Payne et al., 1996b), lesions of all contiguous visual areas prevent intrinsic neuroplastic mechanisms from mediating recovery (Wallace et al., 1989). These cats displayed permanent visual neglect in conjunction with a broader cortical blindness. Finally, in a third group of cats with large unilateral visual cortical lesions (Group C), we reversed the neglect by cooling contralateral parietal cortex or the SC (Lomber et al., 2002). In all cats, we assessed the functional state of the brain by injecting radiolabelled 2-deoxyglucose (2DG) as a measure of neural activity during the cooling-induced neglect (Group A), during the lesion-induced neglect (Group B) or during cooling-induced cancellation of the lesion-induced neglect (Group C). Our results have implications for understanding the syndrome of neglect, and for its treatment.
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Animals and animal care
All cats were purchased from a licensed cat breeder and were group-housed in an enriched environment with a 12 h daylight cycle. Cats were provided with water ad libitum and were fed dry Purina Cat Chow for 1 h per day. Animals were treated according to the National Research Council's Guidelines for the Care and Use of Mammals in Neuroscience and Behavioral Research (2003), and with the approval of the Institutional Animal Care and Use Committee of the Boston University School of Medicine.
Sixteen cats were used in this study. Ten cats were divided into three experimental groups:
Group A: Unilateral cooling loops within the posterior middle suprasylvian (pMS) sulcus (n = 4).
Group B: Unilateral lesion of all contiguous visual areas (n = 3).
Group C: Unilateral lesion of all contiguous visual areas with (i) contralateral pMS cooling loop (n = 2); (ii) contralateral SC cooling loop (n = 1).
Behavioural results for Group C have been reported previously (Lomber et al., 2002), and will be repeated here in textual form. Six additional cats were used as controls: four were unoperated and two additional cats underwent cooling of temporal cortex. This cooling did not have an impact on contralateral structures, and the contracooled hemispheres were included as control data. [Control group same as in Rushmore et al. (2005)].
Cooling loop construction
Cooling loops were constructed from 23 gauge hypodermic tubing (Small Parts, Inc., Miami Lakes, FL, USA) and shaped to conform to the contour of the pMS sulcal cortex or the surface of the SC (Lomber et al., 1999, 2001). A microthermocouple was constructed by soldering the bared ends of twisted copper/constantine wire (30 gauge, Teflon insulated) to the union of the loop using silver solder. The hypodermic tubing was trimmed at the ends and led through a custom-made outside-threaded stainless steel length of tubing. The microthermocouple wire was passed to a thermocouple miniature connector. Dental acrylic was used to hold the hypodermic tubing and the thermocouple wires together, and a threaded cap was screwed on the outside-threaded tubing for protection against inadvertent damage to the hypodermic tubing. Each loop was tested for adequate flow rates and for proper microthermocouple function before sterilization with ethylene oxide.
Surgical procedures
Surgery was performed to implant cooling loops, for unilateral removal of all contiguous visual cortical areas, or both. Surgery employed sterile surgical technique and was carried out in an aseptic environment.
General procedures
On the day before surgery, a catheter was inserted into the cephalic vein under ketamine sedation (10 mg/kg, i.m.). The next day, the cat was anaesthetized with sodium pentobarbital (25 mg/kg i.v., and to effect). Atropine sulphate (0.03 mg/kg, s.c.) was injected to decrease potential alimentary secretions, and dexamethasone sulphate (1.0 mg/kg, i.m.) was given to reduce potential swelling. Antibiotic ophthalmic ointment was applied to prevent corneal drying. The cat's head was secured in a stereotaxic apparatus and centred in Horsley–Clarke coordinates (Reinoso-Suarez, 1961). Core temperature, pulse rate, presence of reflexes and respiratory rate were monitored and anaesthesia adjusted as needed.
Cooling loop implantation
Stainless steel screws (Small Parts, Inc., Miami Lakes, FA, USA.) were tapped into the skull and covered with dental acrylic to provide a base to secure and attach the loop. A cooling loop was selected that best conformed to the individual brain contour and was held in place by dental acrylic. The dura was substituted with Gelfilm (Pharmacia & Upjohn, Kalamazoo, MI, USA), and the bone piece was replaced and covered with dental acrylic.
Removal of contiguous visual areas
Mannitol (1.5 g/kg in 25% aqueous solution; i.v.) was administered to harden the brain and make it more amenable to manipulation. A large unilateral craniotomy was made to expose the marginal, posterolateral, suprasylvian and ectosylvian gyri. Subpial aspiration was used to remove the marginal and suprasylvian gyri to A-P 14, the posterolateral and posterior suprasylvian gyri in their entirety and the cortex in the middle and posterior limbs of the suprasylvian sulcus. Gelatin surgical sponges (Ethicon, Somerville, NJ, USA) were placed in the cortical defect to assist haemostasis and the bone piece was replaced.
Closing and postoperative care
Muscle and skin planes were re-approximated and sutured with chrome gut and silk, respectively. Lidocaine (1%) was injected locally into skin and muscle before suturing.
Cats were placed in a recovery cage with heating pads and were closely monitored. Animals were given fluids (2.5% dextrose in half-strength lactated Ringer solution, s.c.) and antibiotic (AmbipenTM, 300 000 units, i.m.) was administered prophylactically. Buprenorphine (0.01 mg/kg, s.c.) was injected to reduce potential postoperative pain, and soft food and water was provided.
Cats were monitored several times per day and sutures were removed 10 days after surgery. For cats with cooling loops, the margin of the implant was cleaned several times a week with saline and hydrogen peroxide, and topical triple antibiotic ointment was applied to guard against infection.
Perimetry testing
Perimetry testing was performed as described previously (Lomber and Payne, 1996; Payne et al., 1996a; Lomber et al., 2002). The perimetry testing apparatus consisted of a semicircular table (88 cm diameter) with a 30-cm-high wall enclosing the semicircle and holes in the floor at 15° intervals from left 90° to right 90° (Fig. 2A). All testing occurred at photopic illumination levels (85 cd/m2).
Cats were initially trained to fixate a target introduced through the 0° hole (cynosure). The lateral canthi of the eyes were aligned with a line connecting the left 90° and right 90° holes. Upon correct fixation as deemed by the experimenter presenting the fixation target, the cat was allowed to proceed forward and claim a dry food reward at the cynosure. The cat was required to complete 50 consecutive trials to proceed with the next phase of the task. Any non-fixation behaviour in response to target presentation was not rewarded.
Following completion of the fixation task, the full perimetry task was performed. The cat was required to fixate a target at the cynosure. Subsequently, a high-contrast stimulus was introduced through a peripheral opening. A correct response was scored if the cat broke fixation and quickly oriented to the peripheral stimulus. The cat was then allowed to approach the stimulus and was given a high-incentive wet food reward. A failure to detect the peripheral stimulus was signalled by movement of the cat forward to the cynosure, and the cat received a dry, low-incentive food reward. Stimuli were introduced throughout the visual field in a pseudorandom sequence equally balanced for eccentricity and visual hemispace. Slow orienting responses or scanning behaviours were scored as mistrials and not rewarded.
Cats with lesions were tested 3 weeks postoperatively, and at 3-week intervals. No signs of spontaneous recovery were ever noted in any of the three cats. Lesion cats survived for 
3 months after surgery to allow degenerative effects of the lesion to run to completion. Cats with lesion and cooling were tested as detailed in Lomber et al. (2002).
Cooling
Following successful training, cats with cooling loops were acclimatized to wearing a harness connected via a tether to a stage over the perimetry arena. The stage held the cooling pump, a methanol bath and a methanol reservoir. For a cooling experiment, the protective cap was removed from the cooling loop and Teflon tubing was attached to the inlet and outlet of the cooling loop. The tubes were attached to the tether and connected to the cooling circuit. Cats were tested for freedom of movement in the arena, and a pre-cooling set of trials was performed to twice sample every visual field location.
Cooling was effected by circulating chilled methanol through the implanted cooling loop. Room temperature methanol was drawn from the methanol reservoir by a peristaltic pump, circulated through a cold methanol bath and pumped through the cooling loop. The outlet of the cooling loop led to the methanol reservoir. The temperature of the loop depended on the temperature of methanol in the cooling loop, the rate of flow of the methanol and the ability of the brain vasculature to counteract the cooling. Loop temperature was monitored and was changed to the desired temperature by adjusting flow rate. Typically, trials began 5 min after the start of cooling and every visual field position was sampled at least twice.
After the cooling trials were over, the cooling pump was turned off, the coolant tubes were disconnected from the implant, the protective cap reapplied and 5 min allowed before post-cool (re-warm) control data were collected.
2DG administration and euthanasia
For 5–8 days before final procedures, the cat was trained to remain calm for 30–45 min in a nylon veterinary cat sack. On the penultimate day, a venous catheter was inserted into the cephalic vein.
On the day of the 2DG experiment, the cat was put in the nylon sack and the loop was cooled for 5 min to obtain a stable temperature of 1 ± 3°C. The cat was then injected with four boluses of 2-deoxy-[14C]-glucose separated by 5 min (i.v., total 2DG dosage = 100 µCi/kg; specific activity = 310 mCi/mmol) (Vanduffel et al., 1995, 1997, 1998; Rushmore and Payne, 2003; Rushmore et al., 2005). Fifteen minutes later, cats were injected with the anticoagulant heparin (1000 units, i.v.) and the vasodilator sodium nitrite (1% w/v, i.v.), and finally deeply anaesthetized with sodium pentobarbital (65 mg/kg, i.v.). Intact cats and cats with lesions underwent the same procedure without the presence of cooling.
The vascular system of the cat was perfused through the heart with a flush solution [15% sucrose in 0.1 M phosphate buffer (PB), pH = 7.4] for 1 min, followed by a fixative solution (2% paraformaldehyde and 15% sucrose in 0.1 M PB, pH = 7.4) for 5 min. Cooling was discontinued when fixative had entered the brain.
The brain was quickly removed, blocked and photographed. It was covered with albumin and submerged in a –30°C bath of 2-methylbutane for 30 min. The brain was then transferred to a –80°C freezer until cutting.
Histological procedures
Brains were cut into 23-µm thick coronal sections using a Bright OTF cryostat (Hacker Instruments, Inc., Fairfield, NJ, USA) that was set at –23°C. Every fifth section was picked up with a 0.1% gelatin-chrome alum subbed coverslip, heated to 60°C and glued to Bristol board.
Bristol boards with subbed coverslips applied to them were apposed to high-resolution X-ray film (Structurix D7, Agfa, Mortsel, Belgium) along with 14C calibration strips (Amersham, Piscataway, NJ, USA) and exposed at –80°C for 9 days. Films were developed for 10 min (Kodak D19 Developer at 5°C) and fixed (Kodak Fixer) in red light. Sections were transilluminated and digitized (MCID, Imaging Research, Ste. Catherines, Ontario). For each film, points in the calibration curve were joined by linear interpolation and extended with linear extrapolation. Care was taken not to over-expose or approach the saturation level of the film. Distance was calibrated to each image by measuring the number of pixels per millimetre.
Adjacent sections were collected with subbed slides and processed for cytochome oxidase (Wong-Riley, 1979), acetyl thiocholinesterase (Geneser-Jensen and Blackstad, 1971) myelin (Schmued, 1990) or stained for Nissl substance. These sections were used to demarcate extent of primary and secondary degenerative effects of lesion.
Analysis
Lesion/cooling reconstruction
The extent of the cortical defect or of the cooling deactivation was charted onto stereotyped coronal sections through the cat visual cortex and transferred to a whole brain image (Rosenquist, 1985). Secondary independent measures of the lesion were obtained by an assessment of the retrograde degeneration of the lateral geniculate nucleus and of the lateral posterior thalamic nucleus. Degeneration was straightforward to assess because successful lesions induce a retrograde degeneration with profound gliosis and subsequent collapse of the lateral geniculate nucleus, lateral posterior and pulvinar nuclei.
2DG analysis
Two main regions were sampled: the SC and the contralateral pMS cortex. Analysis of the SC was similar to previous approaches (Rushmore and Payne, 2003; Rushmore et al., 2005) and consisted of a mediolateral sampling of 2DG uptake in the following major collicular sublaminae: stratum griseum superficiale (SGS), the stratum opticum (SO), the stratum griseum intermediale (SGI) and the stratum griseum profundum (SGP) (Fig. 1A; Kanaseki and Sprague, 1974). Profiles for each collicular sublamina were obtained by sampling at the midline outside the SC at a point overlying the collicular commissure and then sequentially progressing into the particular sublamina. This approach confers the advantage that each 2DG profile is spatially calibrated and collicular profiles can be overlaid and compared regardless of hemisphere or animal. Also, an impact on specific visual field representation in the SC can be assessed, rather than be diluted by using a gross structural sampling approach. Moreover, this sampling strategy provided embedded control measures of 2DG uptake in extra-collicular sites, which did not change with condition. 2DG profiles from the middle SC were generated from sampling from the middle 10 coronal sections of the SC. For each section, the 2DG profiles were normalized to sectional white matter values (Sharp et al., 1983; Rushmore et al., 2005) and averaged across the 10 sections. This ratio approach controls for variations in glucose uptake between animals, and allowed for a control of intersectional variability.
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Uptake of 2DG in the contracooled pMS sulcus was measured in sections from approximately AP0 to A1 by sequential sampling (250 µm diameter circular samples) from the lateral crest of the posterior aspect of the middle suprasylvian sulcus (MSs) via the fundus to the medial crest, in superficial, middle and deep layers (Fig. 1B). Data from each section were normalized to the white matter average. The average 2DG profiles for each animal were generated from 10 sections.
Significant results between relative 2DG uptake in each neglect group and the normative group were qualified at a P < 0.05 level using non-parametric Mann–Whitney U-tests. Upon comparison of two cases with pMS cooling opposite lesion, we assigned significance at a P < 0.1 level based on the limited sample size. In one instance, we compared the 2DG uptake profile of normal cats with a single case of unilateral SC deactivation opposite a cortical lesion; since comparison of groups was not possible here, we assigned significance when the 2DG uptake values from the deactivation/lesion case exceeded 0.0001 confidence intervals of the control mean.
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Group A: neglect induced by cooling
Behaviour
All four cats demonstrated proficient behaviour in the perimetry task when the cortex was warm. During unilateral cooling of the pMS loop to 0 ± 3°C, cats neglected stimuli moved into the contracooled visual hemispace. Performance in the ipsicooled visual hemispace was unaffected by cooling (e.g. Fig. 2B). These results were comparable with earlier studies using identical techniques (Lomber and Payne, 1996
; Payne et al., 1996a).
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Deactivation
Coronal 2DG radiographs exhibited a decrease in 2DG uptake in the cooled pMS cortex in the vicinity of the loop (Fig. 2C). The extent of deactivation varied from cat to cat but always included the medial bank of the pMS sulcus. In most cats, the effects of cooling was observed to flow dorsally and medially and resulted in diminished 2DG uptake in various portions of the middle suprasylvian gyrus (MSg) (Fig. 3). This has been previously reported following pMS cooling (Lomber et al., 1999
; Lomber and Payne, 2001) and represents an export of the cooling by vascular elements. In two of the cats, the loop caused damage to the pMS cortex (Fig. 3B, iii, iv, black). However, in both instances, the cats were able to perform the orienting task perfectly in the absence of cooling, and cooling deactivation selectively induced deficits in the visual field contralateral to the loop. The patterns of 2DG uptake, therefore, represent the neglect state independent of local damage to the cortex.
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Functional impact of deactivation
1. Ipsilateral SC
The SGS and the SO of the SC receive projections from layer 5 neurons of the pMS sulcus (Segal and Beckstead, 1984
; Harting et al., 1992). Accordingly, cooling silenced neurons in pMS sulcal cortex and resulted in a decrease in 2DG uptake in the ipsilateral SGS and SO layers (Fig. 4). This decrease was relatively localized and on the basis of the visual maps of the SC (Feldon and Kruger, 1970) roughly corresponded to the representations of the area centralis and horizontal meridian, a finding concordant with the retinotopy of cooled pMS cortex (Palmer et al., 1978; Rosenquist, 1985). Thus, as in previous studies (Rushmore and Payne, 2003; Rushmore et al., 2005) the focal decrease in 2DG uptake in the SC was probably communicated via corticotectal projections from the deactivated cortex.
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The pMS cortex sends more limited projections to the ipsilateral intermediate grey level (SGI) of the SC, and no projections to the deep grey lamina (SGP; Harting et al., 1992
). Correspondingly, these layers were largely unaffected by cooling (Fig. 4).
2. Contralateral SC
The contralateral SGS was unaffected by cooling of the pMS sulcus, whereas the SO exhibited an increase in 2DG uptake. 2DG uptake levels in the contralateral SGI and SGP were also significantly higher than normal levels (Fig. 4).
3. Contralateral pMS cortex
Cooling of pMS cortex did not consistently alter 2DG uptake within any layer of the contralateral pMS cortex (Fig. 5). In the middle layers, two points of significant (P < 0.05) deviation from the intact mean were noted, but consistent statistically significant differences were not observed.
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Group B: neglect induced by large cortical lesions
Behaviour
All cats neglected stimuli in the contralesional visual field (Fig. 6A). Orienting to stimuli moved into the ipsilesional visual hemifield was highly proficient. No signs of spontaneous recovery were noted in any of the three cats.
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Lesion
The completeness of each cortical lesion was estimated at the time of brain removal and the lesion borders were subsequently reconstructed from brain sections stained for Nissl substance (Fig. 6B). Also, secondary measures of the lesion size were assessed by examining retrograde degeneration in the thalamus.
In all three cats, the vast majority of intended cortex was judged to be removed and the assessment was confirmed by massive retrograde degeneration in connected thalamic nuclei leading to gliotic infiltration and nuclear collapse. The retrograde degeneration was also evident in 2DG radiographs, which showed a profound reduction in 2DG uptake within the LP/pulvinar complex. A variable amount of spared cortex was present in the most ventral visuotemporal region, which corresponded to neuronal sparing and preserved 2DG signal in the most medial fringe of the lateral posterior nucleus (Fig. 6B, grey). Overall, any cortical sparing was minimal, and is probably irrelevant for the study because none of the cats responded to stimuli presented in the visual hemispace contralateral to the lesion.
Functional impact of lesion
1. Ipsilateral SC
The unilateral ablation of visual cortex resulted in a profound decrease in 2DG uptake in the superficial layers (SGS and SO) of the ipsilateral SC (Fig. 7, left). 2DG uptake values in the SGI were at normal levels. In the deep SGP layer, 2DG uptake was increased at mid-collicular levels.
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2. Contralateral SC
The contralateral SO, SGI and SGP layers displayed heightened 2DG uptake values relative to control measures, but 2DG uptake values in the contralesional SGS were not different from control measures (Fig. 7, right).
3. Contralateral pMS cortex
2DG profiles from the contralateral pMS cortex showed that the pMS cortex contralateral to a large lesion did not differ reliably from normal 2DG uptake profiles (Fig. 8). In the middle layers, two points on the lateral bank were significantly decreased relative to normal values.
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Group C: reversal of neglect by pMS deactivation opposite a large cortical lesion
To this point, we have shown that the presence of neglect for contralateral stimuli, either through unilateral cooling deactivation of pMS cortex (Group A) or from a large unilateral cortical lesion (Group B), corresponds to a specific and bilateral pattern of changes in 2DG uptake in the layers of the superior colliculi (Fig. 9). The pattern was similar in sign and distribution, but not magnitude; the pattern after lesion produced a larger impact in both the ipsilateral and contralateral SC. Regions of the SC in receipt of projections from the lesioned cortex or the deactivated cortex show a decrease in 2DG uptake; these decreases represent the functional removal of efferent projections from the lesioned or deactivated cortex. However, changes observed in the contralateral SC were inconsistent with known direct projections of the resected or deactivated cortex. These changes included heightened 2DG uptake in the contralateral intermediate and deep layers of the SC. Since these alterations cannot result from direct synaptic influence of the deactivated or lesioned cortex, they must represent secondary impacts promulgated by the initial lesion or deactivation.
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These results are in general agreement with theoretical predictions that suggest that the induction of neglect influences crossed circuitry to produce a heightened activation of the contralateral SC (Hilgetag et al., 1999
). To determine whether the heightened activation of the contralateral SC co-varies with the presence of neglect, large unilateral cortical lesions were made to induce the heightened activation of the contralesional SC and induce enduring hemineglect; in the same animals, cooling loops were implanted either in the contralateral pMS cortex (n = 2) or in contact with the contralateral SC (n = 1). The purpose of these loops was to reduce the activity of the SC contralateral to lesion, either directly in the case of the SC loop or indirectly by deactivating the large corticotectal projection from the pMS cortex. The behavioural results and lesion analysis have been reported previously (Lomber et al., 2002), and will be reproduced in textual form below.
Behaviourally, cooling of the pMS loop to 8°C or the SC loop to a temperature of 12°C produced a restoration of orienting responses to stimuli in the formerly blind hemifield (Lomber et al., 2002), which lasted only for the duration of the cooling. The restoration of visual responses in the formerly neglected field disappeared and the neglect became reinstated upon the cessation of cooling (Lomber et al., 2002).
For the final section of the results, we report on the patterns of activity in the SC of the three cats who had an initial lesion-induced neglect for stimuli that was cancelled by subsequent deactivation of either the contralateral pMS cortex or the contralateral SC.
Analysis of 2DG uptake patterns in the SC of cats with restored orienting ability
1. Cooling of pMS cortex opposite a large visual cortex lesion
Deactivation of the pMS cortex opposite a large lesion of visual cortex resulted in a reinstatement of the ability to respond to stimuli in the formerly blind visual field. Analysis of the pattern of 2DG uptake in the SC during the reinstatement indicated that the intermediate and deep layers of the ipsilesional or the ipsicooled SC were not different from 2DG uptake in normal cats (Fig. 10). Thus, the addition of pMS cooling opposite lesion eliminated the lesion-induced heightened activations in the contralesional SO, SGI and SGP, and in the ipsilesional SGP.
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A further comparison between 2DG uptake levels in the SC of lesioned cats and lesioned cats with pMS cooling showed that the SO on the side of the lesion, but not the SGS or SGI, displayed a larger decrease in 2DG uptake than could be accounted for strictly by the impact of the lesion. This difference was not a result of different lesion extents, but a result of the interaction of the pMS cooling with the extant lesion effects. Thus, we conclude that the addition of pMS cooling caused a further decrease in 2DG uptake in the ipsilesional SO, a decrease that was superimposed on the lesion-induced decrease.
In these cases, 2DG uptake in the SGI and the SGP on both sides were comparable both with each other and with levels measured in intact cats. Activity patterns in the SO and SGS were asymmetrical.
2. Cooling of SC opposite a large visual cortex lesion
Similar to the results of adding the contralateral deactivation of pMS cortex to lesion, cooling the SC opposite lesion enabled the animals to respond to stimuli in the contralesional hemifield. Also similar to the effects of the pMS cooling opposite lesion, the addition of the SC cooling eliminated any evidence of heightened activity levels in the contralesional SO, SGI and SGP, and in the ipsilesional SGP (Fig. 11). In addition, direct collicular cooling also induced changes in 2DG uptake throughout the colliculus that were superimposed on the effects of lesion per se. For instance, the decrease in 2DG uptake within the ipsilesional SGS and the SO was clearly accentuated when the SC was cooled opposite lesion. In addition, the ipsilesional SGI measured in lesion cats exhibited no difference from control cats, but displayed a decrease in 2DG uptake when the contralesional SC was cooled (Fig. 12).
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Finally, analysis of relative activity levels showed that SC cooling opposite lesion induced symmetry in the SO lamina, but symmetry was not present, and was even more accentuated in the intermediate (SGI) and deep (SGP) laminae. However, it should be noted that in these layers the asymmetry was between activity levels that were both below normal. In no cases with SC cooling opposite lesion was the activity higher than normal. Thus, it appears that symmetry between the intermediate and deep layers is not necessary for the re-establishment of visual orienting responses.
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Our results show that the presence of neglect, induced either through unilateral cooling of pMS cortex or in conjunction with cortical blindness, is associated with (i) a diminished uptake of 2DG within the superficial laminae of the ipsilateral SC and (ii) a heightened level of 2DG uptake in the intermediate and deeper layers of the contralateral SC. The concordance of collicular 2DG uptake patterns across the two separate groups of animals with similar visual defects lends credence to our claim that neglect is associated with alterations in activity at the level of the SC. Furthermore, when the heightened level of activity in the contralesional SC is diminished, either through deactivation of the colliculus itself or via deactivation of contralateral parietal cortex, the neglect was eliminated. These data indicate that alterations in circuitry in the SC is the major mechanism through which neglect is manifested and reversed and suggest a re-evaluation of current theories of hemispheric asymmetry in the pathogenesis of neglect and in the design and application of therapeutic strategies
Neglect has been understood to be intricately tied to the function of the human cerebral cortex, and in particular, the function of the posterior parietal cortex. However, neglect is not exclusive to the human condition, nor exclusive to insult of the parietal cortical area. Multiple animal models of neglect exist, and visuospatial neglect is also observed following damage to the cingulate cortex, to the superior temporal cortex or to the frontal eye field in most species studied (Mesulam, 1999; Karnath et al., 2004; Lomber and Payne., 2004; Payne and Rushmore, 2004). While the precise symptomology of neglect is likely to vary with cortical site of damage (Mesulam, 1999), the disparate distribution of these neglect-inducing foci suggests that neglect may result not solely from lesion-induced disruption of processing within these locations alone, but that the processing of a more distal and downstream site may serve as a final common structure in the manifestation of neglect. All regions whose disruption induces neglect maintain strong projections to the SC (Harting et al., 1992), and indeed, lesion of SC in humans also produces neglect-like symptoms (Weddell, 2004). As a consequence, the primary induction of neglect may result from a diaschetic impact of lesion along efferent projections to the midbrain. This would explain the convergence of symptoms from disparate cortical regions and would suggest that the SC functions as a final common structure mediating and/or exacerbating the effects of neglect.
In this interpretation, permanent or reversible deactivation of parietal, frontal or cingulate cortical fields results in a condition of reduced excitability in the ipsilesional SC (Payne and Rushmore, 2004). This condition then initiates a disinhibition of the contralateral SC by reducing the activity of intertectal inhibitory fibres. Finally, the disinhibited contralateral SC exerts a higher inhibitory tone on the ipsilesional SC and inhibits it further (Hilgetag et al., 1999; Payne and Rushmore, 2004). Thus, the visual deficit observed after lesion or deactivation of cortex is a consequence of two factors: the removal of parietal cortex processing and the cascade of interactions arising from the subsequent disruption of collicular activity.
The origin of these ideas lies in findings that a cortical lesion-induced visual deficit could be reversed by aspiration lesion of the contralateral SC or after division of the intertectal commissure (Sprague, 1966); to explain these results, Sprague suggested that the SC contralateral to lesion exerted a compounding inhibitory influence on the ipsilesional SC (Sprague, 1966, 1996). Other results suggested that inhibitory influences may arise from an extrinsic source whose axons passed through the superficial layers of the SC to access the intertectal commissure (Wallace et al., 1989). As such, ibotenic acid lesions of the SC, which damaged cells but not axons, were insufficient to reinstate behaviour in the heminanopic field, whereas adding a transection of the caudal intertectal commissure did restore visual orienting (Wallace et al., 1989). Subsequent studies identified the substantia nigra pars reticulata (SNpr; Wallace et al., 1990) and the pendunculopontine tegmentum (PPT; Durmer and Rosenquist, 2001) as providing anatomical sources of the inhibitory influence; ibotenic acid lesions of either structure were sufficient to restore visual orienting following a cortical lesion (Wallace et al., 1990; Durmer and Rosenquist, 2001).
These findings cannot be easily brought to terms with the present behavioural and functional findings (Lomber et al., 2002; present results) and with anatomical results (Appell and Behan, 1990; Olivier et al., 1998; Takahashi et al., 2006). As such, the heightened activation of the contralateral SC may be due to the deactivation of a projection that courses in the intertectal commissure and arises from the ipsilesional SNpr (Harting et al., 1988; Jiang et al., 2003), the contralateral SC (Edwards, 1977; Olivier et al., 1998) or both. In any case, the requirements for involvement are a relatively tight coupling of activity with parietal cortex, a requirement that fits both proposed structures. Unfortunately, we could not examine the SNpr in the current study owing to methodological constraints: neurons of the nigrocollicular pathway are interspersed with other cell types (Jiang et al., 2003), and the presence of the perforating fibres of the cerebral peduncle makes accurate detection of changes difficult at best with the 2DG method. As a consequence, we cannot exclude or include an involvement of the substantia nigra as a component in the functional circuitry.
Regardless, our data show that the presence of neglect is associated with a hyperexcitability in the contralesional SO, SGI and SGP layers of the SC, but not in the SGS layer. In addition, there does not seem to be any lesion-related inhibition of the ipsilateral SC apart from that expected following the functional removal of corticotectal projections; we expected a diminished 2DG uptake signal throughout SGI and SGP, but found none. Moreover, we found that by reducing the hyperexcitability of the contralesional collicular layers, either directly or by deactivating highly connected distant regions, the capacity to respond to moving stimuli in the contralesional visual field was restored.
Unexpected findings in SC patterns after induction of visual deficits
Previous investigation into the functional organization of this system suggested that the initial cortical lesion-induced neglect was maintained and compounded by a suddenly unbalanced midbrain system (Kinsbourne, 1987; Hilgetag et al., 1999; Payne and Rushmore, 2004) via induction of asymmetry between the cortical hemispheres and between the midbrain SC. Our findings provide a test of this idea and force several modifications.
The contralateral pMS cortex
Contrary to speculations that the initial neglect-inducing lesion would reduce or elevate excitability in the homotypic contralateral cortical region and thus on the contralateral SC (Hilgetag et al., 2001; Payne and Rushmore, 2004), we found no consistent or statistically reliable evidence to suggest that the pMS contralateral to lesion or cooling was affected by the deactivation of the contralateral homotypic region. Thus, despite the large anatomical callosal projection (Keller and Innocenti, 1981; Segraves and Rosenquist, 1982), lesion or cooling do not appear to have an impact on the contralateral pMS cortex. These findings are supported by the presence of the multiple kinds of axons in the corpus callosum (Peters et al., 1990) and the spectrum of functional impacts of callosal projections (Payne et al., 1991). Finally, behavioural data from Sherman (Sherman, 1974) indicate that the contralateral cerebral cortex is not necessary for recovery. Overall, our data show that contralateral parietal cortex is not affected by a neglect-inducing manipulation of the contralateral homologue.
The ipsilesional SGI
A strong decrease in 2DG uptake in the ipsilesional/ipsicooled SC was expected on the basis of two arguments. First, we anticipated a functional denervation of cortical inputs would decrease the excitability of superficial collicular layers and vertically telegraph to the intermediate (SGI) and deep (SGP) collicular layers (Behan and Appell, 1992) and render the entire SC hypoexcitable. Second, the impact of a lesion on crossed inhibitory mechanisms between the two SC would be expected to feed back and further inhibit the ipsilesional intermediate and deeper layers (Hilgetag et al., 1999). In the present study, however, we found a decrease in 2DG uptake in the superficial layers SGS and SO, but 2DG uptake in intermediate layers was not different from normal. This constancy of SGI activity confirms previous data that indicate that the superficial layers are functionally isolated from the deeper layers (Wickelgren and Sterling, 1969; Rizzolatti et al., 1970; Stein and Arigbede, 1972; Berman and Cynader, 1975; Ogasawara et al., 1984); alternatively, the normal activity level of the ipsilesional SGI may represent functional compensation by non-visual inputs, which provide substantial inputs to SGI neurons (Huerta and Harting, 1984; Harting et al., 1992; Meredith and Stein, 1993; Harting, 2004).
Experimental models of visual neglect
Both large visual cortical lesions and focal cooling deactivations of pMS cortex are presented as models of neglect, and here we discuss the suitability and limitations of each as appropriate models of neglect. Restricted cooling deactivation of pMS cortex in the cat silences neurons under the cooling loop and produces visual deficits consistent with neglect (Lomber and Payne, 1996, 2004; Payne et al., 1996a). Cooling is a focal manipulation and neurons are functionally silenced, but structurally intact (Lomber, 1999); as such, adaptive or maladaptive mechanisms following cortical damage are not invoked.
Comparable functional deficits cannot simply be created by focal permanent lesions of pMS, since in such cases visual orienting spontaneously recovers within a matter of days, presumably as adjacent cortical regions compensate for the function of the damaged pMS cortex (Payne et al., 1996b). Large lesions incorporating pMS and regions of cortex that potentially compensate are required to produce a stable form of neglect. This model is more similar to the human condition, wherein neglect is elicited by brain damage that is frequently quite extensive, and in which case neglect and hemianopia often co-occur. However, the experimental model involving large cortical lesion suffers from a lack of specificity.
Overall, we are confident that the cooling and lesion approaches are comparable for three reasons. First, the cancellation of visual deficits argues that we are examining recovery from neglect, and not hemianopia, because unlike neglect, hemianopia cannot be reversed—the grain of analysis that striate cortex exerts on the visual field appears irreplaceable. Second, visual deficits induced by a unilateral pMS cooling or by a large visual cortex lesion can both be reversed by cooling precisely the same region of cortex in the intact side (Lomber and Payne, 1996; Lomber et al., 2001). Finally, both approaches produce a pattern on the SC that is similar in sign, but not magnitude, a result concordant with the extent of the cortical manipulation.
We should also note that there is a piece of the puzzle remaining to completely establish this comparison between the two models: the impact of bilateral cooling deactivation of pMS on 2DG uptake in the SC has not yet been performed. While on the basis of the current data and from the data of Lomber and Payne (1996) we would expect the second cooling to reduce 2DG uptake levels to normal or subnormal levels, at this point we can only speculate.
Neglect and its restoration
Our findings suggest that neglect is associated with an increase in excitability in the contralateral SC; when that excitability is diminished, either indirectly through cooling deactivation of the pMS cortex or directly through cooling of the SC, orienting to stimuli in the previously neglected hemifield returns (Fig. 12; Lomber et al., 2002).
These findings are in broad agreement with models of midbrain function (Hilgetag et al., 1999; Payne and Rushmore, 2004), but several features do not reflect theoretical predictions. In particular, we did not find any evidence that the ipsilateral SC is being inhibited beyond the inhibition expected by functional denervation of corticotectal inputs. Indeed, cancelling neglect is associated with a further decrease in 2DG uptake in the ipsilesional SO layer, and unchanged activity profiles in the SGI.
Thus, the simplest explanation we can proffer is that the hyperexcitability of the contralesional SC is sufficient in itself to produce symptoms of neglect. As such, ameliorative manipulations taken previously, including transection of the collicular commissure, ablation of the contralateral SC (Sprague, 1966), as well as the recent findings using cooling (Lomber et al., 2002) were effective because they eliminated the excitability generated in contralateral SC. This mechanism also suggests that therapies that transiently eliminate symptoms of neglect, such as caloric stimulation or neck vibration treatments, may produce their effects by temporarily diminishing excitability in the contralesional SC. It is important to note, however, that this interpretation is germane for the understanding of spatial neglect, but unlikely to account for more complex features or forms of neglect, such as representational neglect (e.g. Bisiach and Luzzatti, 1978).
Summary
We found evidence that the presence of neglect is associated with a specific pattern of activity in the SC, a pattern that indicates a hypoactivity of the layers of the ipsilateral SC receiving input from the deactivation or lesion, and a hyperactivity of the contralateral SC. Depressing the hyperactivity in the intact side with cooling resulted in a cancellation of neglect. Our results encourage a relatively straightforward approach to the treatment of neglect in humans: the moderation of excitability in the contralesional SC. Owing to the deep location of the SC, and the tendency of neglect to naturally ameliorate in humans, an invasive approach is counterindicated. Instead, since we have shown that neglect may be reversed by modulating activity in strongly coupled cerebral structures, it follows that non-invasive methods such as transcranial magnetic stimulation or electrical stimulation may serve to diminish the activity in the SC through long-range connections (Oliveri et al., 2001; Brighina et al., 2003; Valero-Cabre et al., 2005). Obvious candidate regions would be the frontal eye fields and the parietal cortex.
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Footnotes |
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The death of Dr Bertram Payne occurred during the preparation of this manuscript. 
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Acknowledgements |
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We thank the following people who read and made valuable comments on previous versions of this manuscript: Mark Moss, Julie Sandell, Álvaro Pascual-Leone, Alan Peters and Louis Toth. This work was supported by grants NIH NS044624 (R.J.R.), NS032137 and NS47754 (B.R.P., S.G.L. and A.V.C.), NSF IBM9906443 (S.G.L.) and the Wellcome Trust (C.C.H.).
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References |
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