Aging and the rehabilitation of homonymous hemianopia: The efficacy of compensatory eye-movement training techniques and a five-year follow up.

The specificity and effectiveness of eye-movement training to remedy impaired visual exploration and reading with particular consideration of age and co-morbidity was tested in a group of 97 patients with unilateral homonymous hemianopia using a single subject /n-of-1 design. Two groups received either scanning training followed by reading training, or vice versa. The third group acted as a control group and received non-specific detailed advice, followed by training of scanning and reading. Scanning and reading performance was assessed before and after the waiting period, before and after scanning and reading training, and at short-term (11 weeks on average) and long-term follow-up (5 years on average). Improvements after training were practice-dependent and task-specific. Scanning performance improved by ∼40%, reading by ∼45%, and was paralleled by a reduction of subjective complaints. The advice (=control) condition was without effect. All improvements occurred selectively in the training period, not in treatment-free intervals, and persisted in the short- and long-term follow-up over several years. Age had only a minor, although significant effect on improvement in reading after training; co-morbidity had no significant impact on the outcome of training. In conclusion, visual impairments associated with homonymous hemianopia can be successfully and durably reduced by systematic and specific training of compensatory eye-movement strategies. The improvements in compensation strategies were independent of subjects' age and of co-morbidity.

Action blindsight and antipointing in a hemianopic patient.

Blindsight refers to the observation of residual visual abilities in the hemianopic field of patients without a functional V1. Given the within- and between-subject variability in the preserved abilities and the phenomenal experience of blindsight patients, the fine-grained description of the phenomenon is still debated. Here we tested a patient with established "perceptual" and "attentional" blindsight (c.f. Danckert and Rossetti, 2005). Using a pointing paradigm patient MS, who suffers from a complete left homonymous hemianopia, showed clear above chance manual localisation of 'unseen' targets. In addition, target presentations in his blind field led MS, on occasion, to spontaneous responses towards his sighted field. Structural and functional magnetic resonance imaging was conducted to evaluate the magnitude of V1 damage. Results revealed the presence of a calcarine sulcus in both hemispheres, yet his right V1 is reduced, structurally disconnected and shows no fMRI response to visual stimuli. Thus, visual stimulation of his blind field can lead to "action blindsight" and spontaneous antipointing, in absence of a functional right V1. With respect to the antipointing, we suggest that MS may have registered the stimulation and subsequently presumes it must have been in his intact half field.

Translucence perception is not dependent on cortical areas critical for processing colour or texture.

Translucence is an important property of natural materials, and human observers are adept at perceiving changes in translucence. Perceptions of different material properties appear to arise from different cortical regions, and it is therefore plausible that the perception of translucence is dependent on specialised regions, separate from those important for colour and texture processing. To test for anatomical independence between areas necessary for colour, texture and translucence perception we assessed translucency perception in a cortically colour blind observer, who performs at chance on tasks of colour and texture discrimination. Firstly, in order to establish that MS has shown no significant recovery, we assessed his colour perception performance on the Farnsworth-Munsell 100 Hue Test. Secondly, we tested him with two translucence ranking tasks. In one task, stimuli were images of glasses of tea varying in tea strength. In the other, stimuli were glasses of tea varying only in milkiness. MS was able to systematically rank both strength and milkiness, although less consistently than controls, and for tea strength his rankings were in the opposite order. An additional group of controls tested with greyscale versions of the images succeeded at the tasks, albeit slightly less consistently on the milkiness task, showing that the performance of normal observers cannot be transformed into the performance of MS simply by removing colour information from the stimuli. The systematic performance of MS suggests that some aspects of translucence perception do not depend on regions critical for colour and texture processing.

The perception of gloss: a review.

Gloss is a relatively little studied visual property of objects' surfaces. The earliest recorded scientific reference to gloss appears to have been by Ingersoll in 1921: studies at this time were based on the assumption that gloss could be understood as an inherent physical property of a surface, and the priority was to devise a satisfactory method and scale to measure it reliably. As awareness of the complexity of perception grew, efforts were made to distinguish different types of gloss, although these generally still took the form of a search for objective physical measures to be solved within the visual system by means of inverse optics. It became more widely recognised approximately 20 years ago that models of gloss perception based on inverse optics were intractable and failed to explain experimental findings adequately. A temporary decline in the number of published studies followed; however the last decade or so has seen a renewal of interest in the perception of gloss, in an effort to map what is now understood to be a complex interaction of variables including illumination, surface properties and observer. This appears to have been driven by a number of factors, as the study of gloss re-emerged from research into other surface properties such as colour and texture, with technological advances paving the way for new experimental techniques and measurements. This review describes the main strands of research, tracking the changes in approach and theory which have triggered new avenues of research, to the current state of knowledge.

Contrasting the processes of texture segmentation and discrimination with static and phase-reversing stimuli.

Regions of visual texture can be automatically segregated from one another when they abut but also discriminated from one another if they are separated in space or time. A difference in mean orientation between two textures serves to facilitate their segmentation, whereas a difference in orientation variance does not. The present study further supports this notion, by replicating the findings of Wolfson and Landy (1998) in showing that judgments (odd-one-out) made for textures that differ in mean orientation were more accurate (and more rapid) when the textures were abutting than when separated, whereas judgments of variance were made no more accurately for abutting relative to separated textures. Interestingly, however, responses were overall faster for textures differing in variance when they were separated compared to when they were abutting. This is perhaps due to the clear separation boundary, which serves to delineate the regions on which to perform some regional estimation of orientation variance. A second experiment highlights the phase-insensitivity of texture segmentation, in that locating a texture edge (defined by a difference in mean orientation) in high frequency orientation-reversing stimuli can be performed at much higher frequencies than the discrimination of the same regions but with the texture contour masked. Textures that differed in variance did not exhibit this effect. A final experiment demonstrates that the phase-insensitive perception of texture borders improves with eccentric viewing relative to the fovea, whereas perception of the texture regions does not. Together, these experiments show dissociations between edge- and region-based texture analysis mechanisms and suggest a fast, sign-invariant contour extraction system mediating texture segmentation, which may be closely linked to the magnocellular subdivision of visual processing.

Separate channels for processing form, texture, and color: evidence from FMRI adaptation and visual object agnosia.

Previous neuroimaging research suggests that although object shape is analyzed in the lateral occipital cortex, surface properties of objects, such as color and texture, are dealt with in more medial areas, close to the collateral sulcus (CoS). The present study sought to determine whether there is a single medial region concerned with surface properties in general or whether instead there are multiple foci independently extracting different surface properties. We used stimuli varying in their shape, texture, or color, and tested healthy participants and 2 object-agnosic patients, in both a discrimination task and a functional MR adaptation paradigm. We found a double dissociation between medial and lateral occipitotemporal cortices in processing surface (texture or color) versus geometric (shape) properties, respectively. In Experiment 2, we found that the medial occipitotemporal cortex houses separate foci for color (within anterior CoS and lingual gyrus) and texture (caudally within posterior CoS). In addition, we found that areas selective for shape, texture, and color individually were quite distinct from those that respond to all of these features together (shape and texture and color). These latter areas appear to correspond to those associated with the perception of complex stimuli such as faces and places.

Separate processing of texture and form in the ventral stream: evidence from FMRI and visual agnosia.

Real-life visual object recognition requires the processing of more than just geometric (shape, size, and orientation) properties. Surface properties such as color and texture are equally important, particularly for providing information about the material properties of objects. Recent neuroimaging research suggests that geometric and surface properties are dealt with separately within the lateral occipital cortex (LOC) and the collateral sulcus (CoS), respectively. Here we compared objects that differed either in aspect ratio or in surface texture only, keeping all other visual properties constant. Results on brain-intact participants confirmed that surface texture activates an area in the posterior CoS, quite distinct from the area activated by shape within LOC. We also tested 2 patients with visual object agnosia, one of whom (DF) performed well on the texture task but at chance on the shape task, whereas the other (MS) showed the converse pattern. This behavioral double dissociation was matched by a parallel neuroimaging dissociation, with activation in CoS but not LOC in patient DF and activation in LOC but not CoS in patient MS. These data provide presumptive evidence that the areas respectively activated by shape and texture play a causally necessary role in the perceptual discrimination of these features.

Attended but unseen: visual attention is not sufficient for visual awareness.

Does any one psychological process give rise to visual awareness? One candidate is selective attention-when we attend to something it seems we always see it. But if attention can selectively enhance our response to an unseen stimulus then attention cannot be a sufficient precondition for awareness. Kentridge, Heywood & Weiskrantz [Kentridge, R. W., Heywood, C. A., & Weiskrantz, L. (1999). Attention without awareness in blindsight. Proceedings of the Royal Society of London, Series B, 266, 1805-1811; Kentridge, R. W., Heywood, C. A., & Weiskrantz, L. (2004). Spatial attention speeds discrimination without awareness in blindsight. Neuropsychologia, 42, 831-835.] demonstrated just such a dissociation in the blindsight subject GY. Here, we test whether the dissociation generalizes to the normal population. We presented observers with pairs of coloured discs, each masked by the subsequent presentation of a coloured annulus. The discs acted as primes, speeding discrimination of the colour of the annulus when they matched in colour and slowing it when they differed. We show that the location of attention modulated the size of this priming effect. However, the primes were rendered invisible by metacontrast-masking and remained unseen despite being attended. Visual attention could therefore facilitate processing of an invisible target and cannot, therefore, be a sufficient precondition for visual awareness.

Color contrast processing in human striate cortex.

Color constancy refers to the unchanging nature of the perceived color of an object despite considerable variation in the wavelength composition of the light illuminating it. The color contrasts between objects and their backgrounds play a crucial role in color constancy. We tested a patient whose right striate cortex had been removed and demonstrated that he made no use of color contrast in judging color appearance but instead made judgments based simply on wavelength comparison. This was shown by presenting pairs of colored stimuli against a background color that gradually changed across space. When presented with such displays, both normal observers and those with cerebral achromatopsia (cortical color blindness) judge the color appearance of such stimuli on the basis of the chromatic contrast the stimuli make against their background rather than on the physical wavelengths of the light emitted from them. However, our patient made no such use of color contrast but, instead, made color discriminations simply on the basis of wavelength composition. This is consistent with recent findings from monkey electrophysiology that identify cells in early cortical visual areas that signal local contrast and so contribute to the likely mechanism for achieving color constancy.

Covert processing of visual form in the absence of area LO.

The patient D.F., who suffers from severe visual form agnosia, has been found to have a bilateral lesion of area LO, an area known to be intimately involved in the perception of object shape. Despite her perceptual impairment, however, D.F. retains residual form processing abilities that can provide distal visuomotor control, for example in the configuration of her grasp when reaching to pick up objects of different shapes and sizes. This dissociation has been interpreted as reflecting the sparing of a dedicated system for processing the physical properties of objects solely for purposes of guiding action. Here we test this hypothesis in two studies designed to examine whether or not spared shape processing capacities might be revealed under other kinds of indirect test conditions. First, we exploited the fact that a redundant shape cue will speed search for a coloured stimulus within an array, and vice versa. Unlike our control subjects, D.F. showed no facilitation effect of either kind. Second, we used two Stroop tasks in which single coloured uppercase letters were presented. Our intention was to determine (a) whether naming the colour would be influenced by whether the letter was the initial letter of the correct or incorrect colour name (e.g. 'R' or 'G'); and (b) whether the reverse might be true, that is that D.F.'s guesses at letter identity might be influenced by their colour. We found no evidence for a Stroop effect of the former (standard) kind in D.F., but we did find evidence for reverse-Stroop effects. This result may reflect a partial sparing of ventral stream areas specialised for letter-form processing.

Chromatic edges, surfaces and constancies in cerebral achromatopsia.

We tested achromatopsic observer, MS, on a number of tasks to establish the extent to which he can process chromatic contour. Stimuli, specified in terms of cone-contrast, were presented in a three-choice oddity paradigm. First we show that MS is able to discriminate the magnitude of chromatic and luminance contrast, but performance is inferior to that of normal observers. Moreover, MS can discriminate isoluminant borders of different chromatic composition. These abilities are not the result of unintended luminance differences and are abolished when chromatic borders are masked by sharp luminance change. In simple displays, local cone-contrast signals can make a significant contribution to surface colour appearance in normal observers. In more complex displays, the perception of a surface's colour becomes largely independent of the local contrast to its background, via processes presumed to be similar to the edge integration and anchoring stages of Land's Retinex algorithm. We show that in simple displays the percepts of both MS and normal observers are dominated by local chromatic-contrast. But, although the percepts of normal observers change in line with the predictions of retinex theory in more complex displays, those of MS do not, remaining dominated by local contrast signals. We conclude that MS has lost the ability to perform edge integration and that this loss is closely related to his absence of colour experience.

Spatial attention speeds discrimination without awareness in blindsight.

An intimate relationship is often assumed between visual attention and visual awareness. Using a subject, patient GY, with the neurological condition of "blindsight" we show that although attention may be a necessary precursor to visual awareness it is not a sufficient one. Using a Posner endogenous spatial cueing paradigm we showed that the time our subject needed to discriminate the orientation of a stimulus was reduced if he was cued to the location of the stimulus. This reaction-time advantage was obtained without any decrease in discrimination accuracy and cannot therefore be attributed to speed-error trade-off or differences in bias between cued and uncued locations. As a result of his condition GY was not aware of the stimuli to which processing was attentionally facilitated. Attention cannot, therefore be a sufficient condition for awareness.

The primacy of chromatic edge processing in normal and cerebrally achromatopsic subjects.

The local chromatic contrast between surfaces in a visual scene plays an important role in theories of color perception. Our studies of cerebral achromatopsia suggest that this contrast signal is computed independently of the more complex processes such as edge integration and anchoring. We report a study in which we attempted to determine whether local-contrast signals also drove behavior in normal subjects. We sought to reduce the role of edge integration and anchoring by using stimuli whose background varied very gradually in color from top to bottom. The local chromatic contrast of patches relative to such backgrounds depends upon the position at which they are presented. It is therefore possible for patches with identical spectral composition to have opposite contrasts. We constructed stimuli in which two of three vertically arranged discs had the same contrast while the third had opposite contrast. The stimuli were also constructed so that the contrast-odd disc and one of the other two had identical spectral composition while the third disc had different composition. We used these stimuli in an attentional task where, after a brief delay, a letter discrimination target was presented in the location of one of the discs. Attention should automatically be attracted to the odd disc in such a display. Normal observers were faster at making the letter discrimination when the target appeared at the contrast-odd as opposed to spectrally odd location. We conclude that local chromatic contrast, but not raw spectral composition, is accessible to normal observers at an appropriate stage in visual processing to drive attention.

Metacognition and awareness.

It is tempting to assume that metacognitive processes necessarily evoke awareness. We review a number of experiments in which cognitive schema have been shown to develop without awareness. Implicit learning of a novel schema may not involve metacognitive regulation per se. Substitution of one automatic process by another as a result of the inadequacy of the former as circumstances change does, however, clearly involve metacognitive and executive processes of error correction and schema selection. We describe a recently published study in which we serendipitously discovered that a blindsight subject could change the schema with which he processed cue information in orienting spatial attention task without reporting any awareness of this change, or of the cues and targets which respectively directed and were the object his attention.

Attention without awareness in blindsight.

The act of attending has frequently been equated with visual awareness. We examined this relationship in 'blindsight'--a condition in which the latter is absent or diminished as a result of damage to the primary visual cortex. Spatially selective visual attention is demonstrated when information that stimuli are likely to appear at a specific location enhances the speed or accuracy of detection of stimuli subsequently presented at that location. In a blindsight subject, we showed that attention can confer an advantage in processing stimuli presented at an attended location, without those stimuli entering consciousness. Attention could be directed both by symbolic cues in the subject's spared field of vision or cues presented in his blind field. Cues in his blind field were even effective in directing his attention to a second location remote from that at which the cue was presented. These indirect cues were effective whether or not they themselves elicited non-visual awareness. We concluded that the spatial selection of information by an attentional mechanism and its entry into conscious experience cannot be one and the same process.

Effects of temporal cueing on residual visual discrimination in blindsight.

We tested the ability of a blindsight patient, GY, to identify in which of two locations a target was presented in a spatial two-alternative forced choice paradigm (spatial 2AFC). On each trial the subject was asked to make a second manual response indicating whether he had had any awareness of an event occurring during the trial. A cue, presented at the fixation location, could signal the 0.4 s period over which the target appeared within the 10 s duration of each trial. Targets of three contrasts, 93, 43 and 22% were used. We found that GY's ability to discriminate the location of targets in his blind field remained significantly above chance, with and without cueing, for each contrast. Cueing, did, however, significantly improve his performance for low contrast targets. When he performed a similar task with near threshold contrast targets in his spared visual field his discrimination was at chance unless the presentation of targets was cued, despite his reporting more awareness for these stimuli than he did for low-contrast stimuli in his blind field. These results are compared with those previously reported in monkeys who received lesions to their visual cortices as infants or adults. We conclude that (1) GY's blindsight is qualitatively different from near-threshold normal vision. (2) In common with infant-lesioned monkeys his blindsight remains even in the absence of temporal cues. (3) Residual vision is subject to modulation by attentional processes, or arousal, associated with temporal cueing.

Form and motion from colour in cerebral achromatopsia.

Patients with cerebral achromatopsia, resulting from damage to ventromedial occipital cortex, cannot chromatically order, or discriminate, hue. Nevertheless, their chromatic contrast sensitivity can be indistinguishable from that of normal observers. A possible contributor to the detectability of chromatic gratings is the subadditive nature of certain colour combination such that mixtures of, for example, red and green (yielding yellow) appear dimmer than expected from the simple addition of luminances. This subadditivity is believed to reflect colour-opponent interactions between the outputs of long- and medium-wavelength cones. We performed a first-order compensation for such subadditivity in chromatic gratings and demonstrated that their detection was still not abolished in an achromatopsic patient. In addition, we used a two-alternative forced-choice procedure with an achromatopsic patient, who was required to judge the apparent relative velocity of two drifting gratings with different degrees of compensation for subadditivity. It is well known that isoluminant gratings, constructed by adding a red and green sinusoidal grating of identical peak luminances in antiphase, appear to drift substantially slower than an achromatic grating with the same velocity. Adding 2f luminance compensation to an isoluminant grating of spatial frequency f, resulted in an identical minimum of perceived velocity at a compensation contrast of 5% in both achromatopsics and normal observers. Furthermore, while compensation for subadditivity did not substantially compromise grating detection at low contrasts, such correction severely affected motion detection. Saccadic eye movement accuracy and latency were also measured to uncompensated chromatic, compensated chromatic and achromatic targets. We conclude first that subadditivity, resulting from colour-opponent P-channel processes, influences motion judgements. The ability to extract motion from chromatic differences alone is little, if at all, different in achromatopsic and normal vision. Second, the paradoxical detection of sinusoidally modulated chromatic gratings in achromatopsic patients is not merely a result of subadditivity. Third, saccadic latency, but not accuracy, to chromatic targets is affected by luminance compensation. Finally, and more generally, wavelength processing continues to contribute to several aspects of visual processing even when colour is not perceived.

Cortical color blindness is not "blindsight for color".

Cortical color blindness, or cerebral achromatopsia, has been likened by some authors to "blindsight" for color or an instance of "covert" processing of color. Recently, it has been shown that, although such patients are unable to identify or discriminate hue differences, they nevertheless show a striking ability to process wavelength differences, which can result in preserved sensitivity to chromatic contrast and motion in equiluminant displays. Moreover, visually evoked cortical potentials can still be elicited in response to chromatic stimuli. We suggest that these demonstrations reveal intact residual processes rather than the operation of covert processes, where proficient performance is accompanied by a denial of phenomenal awareness. We sought evidence for such covert processes by conducting appropriate tests on achromatopsic subject M.S. An "indirect" test entailing measurement of reaction times for letter identification failed to reveal covert color processes. In contrast, in a forced choice oddity task for color, M.S. was unable to verbally indicate the position of the different color, but was surprisingly adept at making an appropriate eye movement to its location. This "direct" test thus revealed the possible covert use of chromatic differences.

Residual Vision in Multiple Retinal Locations within a Scotoma: Implications for Blindsight.

There is an important new proposal that "blindsight"-the ability to detect and identify visual stimuli by forcedchoice guessing and in the absence of conscious awareness when they fall in blind regions of the visual field-is a function of residual "islands" of undamaged visual cortex. This stands in contrast to the widely accepted view that blindsight is exclusively a function of secondary visual pathways. According to the new view, residual vision in blindsight should be patchy. Thus, when apparently wide areas of residual vision in blindsight are found, these may be due to eye-movements that allow stimuli to pass over retinal locations corresponding to islands of sparing. We tested this hypothesis by examining the distribution of residual vision in blindsight when the effects of eye movements on the retinal location of stimuli were minimized. We report a series of experiments that examined twealternate forcedchoice discrimination in the blind field of the subject GY. Using a dual-Purkinje image eye-tracker we applied three methods of minimizing the effects of retinal slippage due to eye-movements on discrimination performance: fixation stability-dependent trials, software image stabilization, and post hoc rejection of trials in which saccadic eye-movements were detected. In the first experiment, GY's discrimination performance was significantly above chance in 8 of 15 locations tested. In the subsequent experiments the subject knew the location of the target in each block of trials, and this resulted in improvements to performance in a further three locations. Increasing the luminance of the stimulus display (while maintaining 95% target contrast), and increasing the temporal discriminability of the forced choice produced performance above chance in all but two of the locations tested. The consistent chance performance observed in two locations in the lower visual field nevertheless implies that GY's blindsight does not extend over the whole of his scotoma. Nevertheless, abolishing, or minimizing, the effects of eye-movements did not result in a loss of detection in all the widely separated regions tested, and we thus conclude that GY's blindsight cannot adequately be explained in terms of islands of spared vision. Islands may account for residual vision in scotomata in some patients, but cannot be a universal account of the phenomenon of blindsight.