Brain, Vol. 122, No. 8, 1533-1538,
August 1999
© 1999 Oxford University Press
Differential pupillary constriction and awareness in the absence of striate cortex
1 Department of Experimental Psychology, University of Oxford and 2 Applied Vision Research Centre,City University, London, UK
Correspondence to:
Professor L. Weiskrantz, Department of Experimental Psychology, University of Oxford, South Parks Road, Oxford OX1 3UD, UK
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Abstract |
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The fact that the pupil constricts differentially to visual stimuli in the absence of changes in light energy makes it a valuable tool for studying normal function as well as residual capacity in hemianopic subjects. When pupillometrically effective stimuli such as equiluminant gratings or coloured patches with an abrupt onset and offset are presented to the `blind' hemifield, a hemianopic subject with damage largely restricted to striate cortex (V1) sometimes reports being `aware' of the transient onset/offset, although without `seeing' as such. The question addressed here is whether the pupil still responds in the condition of blindsight in its strict sense—i.e. discriminative capacity in the absence of acknowledged awareness—when stimuli are deliberately designed to eliminate awareness. This was accomplished by making stimulus onset and offset slow and gradual. The results with a well-studied hemianope, G.Y., demonstrate that there is still a pupillary constriction to isoluminant achromatic gratings and red-coloured stimuli, although reduced in size, in the absence of acknowledged awareness.
pupillometry; visual cortex; blindsight; residual vision; visual awareness
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Introduction |
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The pupil of the normal human eye constricts in response not only to increases or decreases in light energy, but also selectively to changes in the spatial structure of a visual stimulus even in the absence of a change in mean retinal illuminance. For example, it responds differentially to achromatic gratings, depending on their spatial frequency, with a sensitivity that resembles the normal contrast sensitivity function; it also responds to the onset of coherent motion or colour changes even when luminance contrast changes are not involved (Barbur, 1995
).
The pupillary measure, which is criterion-independent and non-verbal, lends itself especially to the systematic measurement of residual visual capacity in subjects, both human and animal, with visual cortex damage and other conditions. A previous study has shown that in the absence of striate cortex (V1) there is a reliable (but reduced) pupillary constriction response to gratings in the `blind' hemifields of both monkey and human (Weiskrantz et al., 1998). The function is narrowly tuned, with a peak at ~1 cycle/°, and a maximum cut-off (acuity) of ~7 cycles/°, which is a reduction in acuity of ~2 octaves compared with the sighted field. The shape and limits of the function correlate very well with those obtained psychophysically with two alternative, forced-choice discriminations between gratings of varying spatial frequency and luminance-matched homogeneous stimuli.
The sine-wave grating stimuli that were used to establish these phenomena all had abrupt temporal onsets and offsets. Such rapid transients are frequently associated, at least in one of the human subjects who took part in the studies, with a `feeling' or `knowing' that some event has occurred, although he cannot identify the event and does not `see' it. In the absence of such transients, psychophysical discriminations in the blind field can still be possible, but they lack accompanying awareness. The question that arises, therefore, is whether the pupillary constriction reflects, and is restricted to, the conditions that generate such reports of `awareness' or `feelings'. As the pupil is partly under the control of the autonomic nervous system, its responses are perhaps associated with an alerting by events of special importance or danger, namely those that move rapidly or appear suddenly. It is possible that the pupillary response to such stimuli might always be associated with the sense of `awareness' of events occurring in the `blind' hemifield. If that were the case, then the special advantages of using the pupil as a measure of residual function would not apply to the study of `blindsight' as strictly defined—sensitivity in the absence of any acknowledged awareness—found by other methods, e.g. forced-choice guessing, pointing, in subjects with damage to V1.
The purpose of the present study, as we foretold in our comparative paper (Weiskrantz et al., 1998), was to investigate whether the pupil constricts to stimuli in the `blind' hemifield even when they are designed to eliminate `awareness', i.e. whether it is a necessary condition for the occurrence of a pupillary constriction to equiluminant stimuli in the blind field that the subject also has an experience of the occurrence of the events. Awareness can be eliminated or made very unlikely by changing particular stimulus parameters, such as contrast (Weiskrantz, 1986; Weiskrantz et al., 1995), temporal onset or speed. In the present study we used `ramped' stimuli with gradual onset and offset and compared them with stimuli with a stepped, abrupt temporal profile.
If the pupil still responds selectively to spatial structure and spectral composition (among other attributes) in the absence of awareness, it could offer a convenient method of screening for the presence of residual visual function in hemianopia associated with visual cortical damage. In the clinical situation most hemianopes deny having visual awareness of stimuli directed to the blind hemifield, but there are good reasons for supposing that residual function may still yet exist based on processing by pathways that bypass striate cortex (Cowey and Stoerig, 1991). It is difficult in practical terms to screen by `forced-choice guessing' psychophysical techniques, which may be why it is still uncertain what the incidence or the character of residual function is in the hemianopic clinical population.
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Subject
G.Y. is a 41-year-old man whose left occipital cortex was damaged in a motor car accident when he was 8 years old. He has a right homonymous hemianopia with macular sparing of 3° extending into his otherwise blind hemifield. An MRI scan of the lesion appears in Barbur et al. (Barbur et al., 1993
) and shows that he has almost total destruction of striate cortex (V1) with little damage to extrastriate visual areas apart from V2. He has been involved in several investigations (Barbur et al., 1980, 1994a; Blythe et al., 1986, 1987; Weiskrantz et al., 1991, 1995; Brent et al., 1994; Azzopardi and Cowey, 1997) that demonstrate a capacity to detect, localize and discriminate stimuli in his blind hemifield. G.Y. gave informed consent to participate in the experiments which were approved by the Ethics Committee of the Department of Experimental Psychology, University of Oxford.
Stimuli
Monochromatic sinusoidal gratings
All gratings were 12° x 12°, arranged in a square, displayed on a VDU 23.5° x 18° high in the Oxford laboratory. The bars of the grating were vertically oriented; the lowest edge of the grating was aligned with the subject's horizontal meridian. The nearest edge of the gratings was 8° from his vertical meridian. Most gratings were of 1 cycle/°, but there were also some at 3 and 5 cycles/°. The stimuli were presented on a high resolution display monitor that was calibrated for the spectral radiance and the luminance versus applied voltage of each phosphor. The background luminance of the monitor equalled the average luminance of the gratings, and was set to 10 cd/m2 and CIE (Commission Internationale de l'Eclairage) = (x, y) chromaticity 0.308, 0.309, calibrated with a Minolta CS100 chromameter (Milton Keynes, UK). In the `unramped' condition, the gratings had a stepped, abrupt onset and offset. Their duration was 1000 ms. In the `ramped' condition there was a gradual linear increase and decrease of contrast. Each such ramp lasted 500 ms. In most ramped conditions the grating remained at maximum contrast for 500 ms, but in some tests this was 1000 ms. Gratings were presented in blocks of 25 trials, with parameters held constant within each block.
Red-coloured stimuli
Coloured patches of various sizes were used, all positioned such that the nearest edge was at least 4.5° away from the vertical meridian. Testing was carried out in the City University laboratory. Two-dimensional eye-movements were measured with each pupil recording and these data provided a measure of the fixation stability of the eye. The coloured stimulus was generated on a random background field of luminance 12 cd/m2 and CIE = (x, y) chromaticity 0.287, 0.3. Full spectral calibration of the display for each of the three phosphors made it possible to generate photopically isoluminant stimuli that also had zero scotopic (i.e. rod) contrast. For the background chromaticity selected, a direction of chromatic displacement that satisfied the d-isoluminant condition was possible towards the red/violet region of the spectrum locus. The use of d-isoluminant stimuli ensures that any rod-driven light reflex is minimized when a photopically isoluminant chromatic stimulus is presented to the eye.
The uniform background field subtended a visual angle of 29° x 23°. The display was viewed through a large infrared reflecting mirror that makes it possible to image the pupil of the eye (Barbur et al., 1987). A tunnel-like enclosure that was covered with black velvet absorber connected the display to the infrared reflecting mirror. This arrangement ensured that no light from the display could illuminate surrounding objects and that the light reaching the subjects from the walls of the enclosure was minimized.
The red stimulus was presented for 1 s and its chromaticity was ramped either suddenly or gradually from 0 to a maximum of 0.12 units (as measured in the CIE x, y diagram). In experiments with ramped stimuli, the change in chromaticity was changed up and down gradually over 428 ms according to a half cycle sinusoid. The order of presentation of the stimuli was randomized; 48 pupil traces were averaged for each stimulus.
Measurement
Pupillary constriction was measured by P_Scan software (City University, London, UK), which also provides continuous monitoring of accuracy and reliability of eye fixation (Barbur et al., 1987). All records contaminated by eye movements or blinks were rejected.
G.Y. has been well trained to register `commentary responses' after stimuli delivered to his blind hemifield, in which he indicates whether he was `aware' or `unaware' of the stimulus, and to follow the instruction that `unaware' means no `awareness', `feeling' or `knowing' of any kind. In the experiment with gratings he was asked to keep a running record and report the frequency after each block of 25 trials. In the experiments with red stimuli, G.Y. made a verbal `commentary' response after every trial, either `aware' or `not aware', and these were recorded. It should be noted that, unless otherwise qualified, the judgement `aware' does not mean having a visual percept; it is an `awareness', `feeling' or `knowing' that an event has occurred, without `seeing', as such.
Procedure
Grating experiments
Each trial started with a manual response from the subject indicating `ready'. If the image of the pupil and visual fixation as seen on a monitor were stable, the trial proceeded. There was a variable interval of between 500 ms and 4 s, after which the stimulus was presented, ensuring that there was no fixed temporal interval between the start of the trial and the delivery of the stimulus. Approximately 2 s after the end of the stimulus, when the P_Scan processing was complete, an auditory signal informed G.Y. that he could blink or move his eyes until he, again, gave a ready signal for the next trial. Thus, the intertrial interval also was variable, but averaged ~5 s.
Coloured stimulus experiments
Each trial was started by the experimenter when pupil size and fixation on a monitor were seen to be stable. The time interval between successive trials was random in the range of 2–5 s. There was an auditory signal indicating the end of the processing, approximately 2 s after the end of the stimulus. At that point, G.Y. gave a verbal commentary response, but he could blink and move his eyes before giving the response. Once he gave that response, the next trial was started.
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Ramped gratings
There were 15 blocks each of 25 trials using a grating of 1 cycle/° and Michaelson contrast of 0.8. For 13 blocks, G.Y. reported no awareness whatever. After one block he commented `hint of something in six or seven trials' and in another block he reported awareness of three trials. Thus, out of 375 trials, he indicated the absence of awareness in 364 or 365. The small proportion of positive responses (2.9%) is of the same order of magnitude as false positive responses to blank stimuli (2%) found in another study of the same subject (R. Kentridge, unpublished results). In three of the 15 blocks the maximum stimulus level was maintained for 1000 ms, instead of 500 ms. In both of these blocks he, likewise, reported no awareness. There were two blocks with a stimulus of 3 cycles/°, and two with 5 cycles/°. In none of these did he report awareness.
There was a small but reliable pupillary constriction associated with the ramped trials for the 1 cycle/° stimulus of 500 ms duration, and a slightly larger one for the stimulus of 1000 ms duration (Fig. 1B and C), both of them significant (sign test B: z = 3.8, P < 0.001; C: z = 3.64, P < 0.001). In order to determine the change objectively we measured for each trial the mean pupillary diameter during the 400 ms (20 bins) straddling the onset of the ramped stimulus and the 400 ms straddling the point 1100 ms after the stimulus onset. The second mean was then rated as lower or higher than the first. There was no significant response (mean change <0.25%) to gratings of 3 or 5 cycles/°.
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Unramped gratings
There were seven blocks of 25 trials each using a stimulus of 1 s and 1 cycle/°. G.Y. reported awareness of every trial in three blocks, and in the great majority in the remaining blocks. Out of 175 trials, he reported awareness in 169. There were two blocks for 3 cycles/°; he reported awareness for 25 of the 50 trials. There were two blocks at 5 cycles/°, when he reported no awareness, but with a small yet reliable pupillary response. The pupillary response (Fig. 1A
) was larger than for ramped stimuli (also highly significant, P < 0.001) and of the same order of magnitude previously found for G.Y. using unramped gratings (Weiskrantz et al., 1998).
A summary of the results with ramped and unramped grating stimuli is shown in Table 1. The results in the table have been converted to percentage changes for purposes of comparison.
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Coloured stimuli
The results for the ramped versus unramped stimuli are shown in Fig. 2
. For the unramped condition G.Y. reported `aware' on 46 out of 48 trials. In the ramped condition, it was six out of 48 trials. Both the ramped and unramped stimuli yielded significant pupil responses that cannot be attributed to pupil noise fluctuations. Even the smaller of the two constrictions, 0.95 mm (the ramped condition), is highly significant (P < 0.001, two-tailed t test). There was a significant difference between the amplitudes of the ramped versus unramped trials (one-tailed t test, comparing means and standard errors, P < 0.05). As the ramped condition produced some `aware' responses, one should consider whether they alone could account for the pupillary results. If it were assumed that the 42 out of 48 `unaware' trials yielded no change in the ramped condition, then for the six `aware' trials to generate the observed mean change over the 48 trials, they would have been required to have a constriction of 0.8 mm each, which is considerably larger than the amplitude measured even in the intact field to this stimulus. On the other hand, if it were assumed that all `aware' trials yielded a mean constriction equal to the observed mean and that the remaining `unaware' traces yielded no constriction, the expected averaged response would be as shown by the dotted curve in Fig. 2. The amplitude of the predicted constriction is about nine times smaller than that observed experimentally. It is reasonable to conclude, therefore, that the result for the ramped trials with red stimuli demonstrates a clear pupillary change in the absence of reported awareness by G.Y.
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Discussion |
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Regarding the main question addressed in this study, the conclusion is that pupillary responses can be demonstrated to gratings of equal space-averaged luminance and of a coloured stimulus even when parameters are arranged so as to eliminate acknowledged awareness of them in G.Y.'s `blind' hemifield. The pupillary methodology, therefore, is applicable to the study and characterization of residual visual function in the original strict definition of blindsight—residual visual function without acknowledged awareness (Weiskrantz, 1986
). Evidence of robust colour responses by two hemianopic subjects, in the absence of acknowledged awareness, will be included in a fuller study of pupillary responses to colour (J. L. Barbur and L. Weiskrantz, in preparation), but which further reinforces the present conclusion.
In the present study we used only a binary scale for indicating awareness and unawareness, and it might be argued that a multi-point scale would give a more sensitive measure. However, G.Y. is well practised in the use of the binary scale, and when his use of this was compared directly with a multi-point scale the result was that a `zero' judgement was equivalent with both measures (Sahraie et al., 1998; also summarized in Weiskrantz, 1997, pp. 248–9), i.e. for him `unaware' means precisely that.
The finding that a pupillary response can be obtained in the absence of acknowledged awareness could be of considerable importance from a practical point of view. Notwithstanding a subject's denial of seeing in the hemianopic field, pupillometry may reveal residual function for colour and/or for spatial structure, necessarily mediated via extra-striate pathways when V1 is damaged. Any such evidence could then lead to further examination by psychophysical methods, combined with rehabilitation procedures (Zihl and von Cramon, 1985; Kerkhoff et al. 1994; Kasten and Sabel, 1995).
Another practical consideration concerns the uncertainty regarding the actual incidence of blindsight, in part due to the vagaries of lesion size and disposition, but perhaps also in part because of the difficulty of `guessing' about `unseen' stimuli. Pupillometry is an indirect method that is unencumbered with the difficulties of verbal instructions in blindsight, e.g. when subjects are urged to `guess' about stimuli they might strongly protest they cannot see. Many subjects (and no doubt some experimenters as well) find the demands excessively counter-intuitive, and indeed some subjects simply refuse to participate. Pupillometry, like other indirect methods (reviews by Weiskrantz, 1990; Stoerig and Cowey, 1997), can be used to assess residual function independently of such demands upon the subject and is also criterion-free in signal detection terms.
It also has the possibility of revealing the detailed quantitative spatiotemporal properties of the visual `channels' that remain in the absence of primary visual cortex, even when there is no awareness of the stimuli. Such indirect methods require validation with psychophysical methods if inferences are sought about discriminative capacity as such, but such a validation between pupillometric and psychophysical measures has been established both for gratings (Weiskrantz et al., 1998) and for colour (Barbur et al., 1992, 1994b) for this particular blindsight subject.
While a pupillary constriction occurs in the absence of acknowledged awareness, it is reduced in magnitude in relation to those conditions that yield awareness (albeit without `seeing'). There appears to be a correlation between frequency of reports of awareness and the size of the pupillary response for gratings and coloured stimuli in the blind field stimulus, which perhaps is true for all categories of effective visual events. This might lead to a conclusion that the `unawareness' state is qualitatively continuous with, but quantitatively weaker than, the state in which there is reported awareness. The question of whether blindsight—discrimination without awareness—is merely a quantitative degradation of normal vision, or is qualitatively distinct (as the evidence suggests), is discussed elsewhere (Azzopardi and Cowey, 1997, 1998; Weiskrantz, 1997). But as regards the pupillary response, while it may well be a component of a general defensive and alerting reaction to particular effective salient stimuli, awareness is not a necessary accompaniment, at least for events in the blind field. On the other hand, when the subject does report awareness of a visual event in the blind field (despite not seeing anything and in the absence of any change in energy level), it is likely that an accompanying pupillary constriction does occur.
The results have interesting implications. The subject's pupil only responds to particular properties of visual stimuli, regardless of whether he reports any awareness of them. For there to be reported awareness of equiluminant stimulus changes, it is apparently important that there be a transient component in the stimulus (or possibly some other feature lending it high salience). Nevertheless, such a component is not sufficient, because the actual visual content of the equiluminant stimuli is also important. For example, G.Y. does not report awareness to an abrupt onset equiluminant sine-wave grating with a spatial frequency outside the range of the psychophysical and pupillometric sensitivity of the blind field (but within the range of his normal hemifield), nor does he report awareness of the colour of an abruptly presented red stimulus (Brent et al., 1994). Even when he does report awareness in response to the presentation of a grating or a coloured stimulus, it is only the transient onset or offset of which he is aware. G.Y. never reports any awareness of bars of the gratings as such, nor of the red colour as such. His residual capacity depends on content, but it is perceptually contentless.
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Acknowledgments |
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We wish to thank G.Y. once again for his patient co-operation, and Carolyne Le Mare and Lynda Irving-Bell for their assistance with testing and data analysis. We also thank Paul Azzopardi for preparing the software used to generate ramped gratings. The research was supported by MRC project grant G9607833 to L.W. and J.B. and by MRC programme grant G97/397/B to A.C.
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Received January 15, 1999. Revised March 9, 1999. Accepted March 12, 1999.
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