Arash Sahraie

Oculomotor control and the maintenance of spatially and temporally distributed events in visuo-spatial working memory

Previous studies have demonstrated that working memory for spatial location can be significantly disrupted by concurrent eye or limb movement (Baddeley, 1986; Smyth, Pearson, & Pendleton, 1988). Shifts in attention alone can also interfere with spatial span (Smyth & Scholey, 1994), even with no corresponding movement of the eyes or limbs (Smyth, 1996). What is not clear from these studies is how comparable is the magnitude of effect caused by different forms of spatial disrupter. Recently, it has been demonstrated that limb movements produce as much interference with spatial span as do reflexive saccades (Lawrence, Myerson, Oonk, & Abrams, 2001). In turn this has led to the hypothesis that all spatially directed movement can produce similar effects in visuo-spatial working memory. This paper reports the results of five experiments that have contrasted the effect of concurrent eye movement, limb movement, and covert attention shifts on participants’ working memory for sequences of locations. All conditions involving concurrent eye movement produced significantly greater reduction in span than equivalent limb movement or covert attention shifts with eyes fixated. It is argued that these results demonstrate a crucial role for oculomotor control processes during the rehearsal of location-specific representations in working memory.

Increased sensitivity after repeated stimulation of residual spatial channels in blindsight

Lesions of the occipital cortex result in areas of cortical blindness affecting the corresponding regions of the patients visual field. The traditional view is that, aside from some spontaneous recovery in the first few months after the damage, when acute effects have subsided the areas of blindness are absolute and permanent. It has been found, however, that within such field defects some residual visual capacities may persist in the absence of acknowledged awareness by the subject (blindsight type 1) or impaired awareness (type 2). Neuronal pathways mediating blindsight have a specific and narrow spatial and temporal bandwidth. A group of cortically blind patients (n = 12) carried out a daily detection “training” task over a 3-month period, discriminating grating visual stimuli optimally configured for blindsight from homogeneous luminance-matched stimuli. No feedback was given during the training. Assessment of training was by psychophysical measurements carried out before and after training and included detection of a range of spatial frequencies (0.5–7 cycles per degree), contrast detection at 1 cycle per degree, clinical perimetry, and subjective estimates of visual field defect. The results show that repeated stimulation by appropriate visual stimuli can result in improvements in visual sensitivities in the very depths of the field defect.

Awareness of Faces Is Modulated by Their Emotional Meaning

A central question in perception is how stimuli are selected for access to awareness. This study investigated the impact of emotional meaning on detection of faces using the attention blink paradigm. Experiment 1 showed that fearful faces were detected more frequently than neutral faces, and Experiment 2 revealed preferential detection of fearful faces compared with happy faces. To rule out image artifacts as a cause for these results, Experiment 3 manipulated the emotional meaning of neutral faces through fear conditioning and showed a selective increase in detection of conditioned faces. These results extend previous reports of preferential detection of emotional words or schematic objects and suggest that fear conditioning can modulate detection of formerly neutral stimuli.

Intact navigation skills after bilateral loss of striate cortex

A patient with bilateral damage to primary visual (striated) cortex has provided the opportunity to assess just what visual capacities are possible in the absence of geniculo-striate pathways. Patient TN suffered two strokes in succession, lesioning each visual cortex in turn and causing clinical blindness over his whole visual field. Functional and anatomical brain imaging assessments showed that TN completely lacks any functional visual cortex. We report here that, among other retained abilities, he can successfully navigate down the extent of a long corridor in which various barriers were placed. A video recording shows him skillfully avoiding and turning around the blockages. This demonstrates that extra-striate pathways in humans can sustain sophisticated visuo-spatial skills in the absence of perceptual awareness, akin to what has been previously reported in monkeys. It remains to be determined which of the several extra-striate pathways account for TNs intact navigation skills.

Orienting to threat: faster localization of fearful facial expressions and body postures revealed by saccadic eye movements

Most studies investigating speeded orientation towards threat have used manual responses. By measuring orienting behaviour using eye movements a more direct and ecologically valid measure of attention can be made. Here, we used a forced-choice saccadic and manual localization task to investigate the speed of discrimination for fearful and neutral body and face images. Fearful/neutral body or face pairs were bilaterally presented for either 20 or 500 ms. Results showed faster saccadic orienting to fearful body and face emotions compared with neutral only at the shortest presentation time (20 ms). For manual responses, faster discrimination of fearful bodies and faces was observed only at the longest duration (500 ms). More errors were made when localizing neutral targets, suggesting that fearful bodies and faces may have captured attention automatically. Results were not attributable to low-level image properties as no threat bias, in terms of reaction time or accuracy, was observed for inverted presentation. Taken together, the results suggest faster localization of threat conveyed both by the face and the body within the oculomotor system. In addition, enhanced detection of fearful body postures suggests that we can readily recognize threat-related information conveyed by body postures in the absence of any face cues.

Acquired colour vision defects in glaucoma-their detection and clinical significance.

Colour vision defects associated with ocular disease have been reported since the 17th century. Kollner1 in 1912 wrote an acute description of the progressive nature of colour vision loss secondary to ocular disease, dividing defects into “blue-yellow” and “progressive red-green blindness”.2 This classification has become known as Kollners rule, although it is often imprecisely stated as “patients with retinal disease develop blue-yellow discrimination loss, whereas optic nerve disease causes red-green discrimination loss”. Exceptions to Kollners rule3 4 include some optic nerve diseases, notably glaucoma, which are primarily associated with blue-yellow defects, and also some retinal disorders such as central cone degeneration which may result in red-green defects. Indeed, in some cases, there might be a non-specific chromatic loss. Colour vision defects in glaucoma have been described since 18835 and although many early investigations indicated that red-green defects accompanied glaucomatous optic neuropathy,1 later studies suggested that tritan defects predominate.6-9 This change of view largely reflected improved experimental design over time—in early studies no distinction was made between the various types of glaucoma, nor was any allowance usually made for the age distribution of subjects in the sample. A summary of the early research can be found in Drance et al .7 Although modern studies control for confounding factors such as increasing lens density and decreasing pupil size with age, some controversy still surrounds the nature of colour vision defects in primary open angle glaucoma (POAG). Comparing previous research evaluating chromatic discrimination in POAG is complicated by wide variations in methodology and experimental conditions. In recent years, computer generated colour tests have provided the means for isolating the processing of chromatic signals from the accompanying achromatic cues.10-15 In these tests, typically, the subject is asked to report the presence of a coloured target such …

Minimum presentation time for masked facial expression discrimination

Backward masking is a popular method of preventing awareness of facial expressions, but concerns have been expressed as to the effectiveness of masking in previous research, which may have resulted in unjustified claims of unconscious processing. We examined the minimum presentation time for discrimination of fearful, angry, happy and neutral faces in a backward masking task using both objective sensitivity measures, based on signal detection analysis, and subjective awareness ratings. Results from two experiments showed for all expressions the mean sensitivity and the sensitivity scores of most individual participants were above chance at presentation times of 20 ms. Awareness ratings for happy, fearful and angry also exceeded baseline ratings from 20 ms onwards. Overall sensitivity in both experiments was greatest for happy expressions, which is an agreement with previous reports. The results support the possibility of incomplete masking in earlier studies that used masking to prevent awareness of facial expressions.

Spatial channels of visual processing in cortical blindness

Blindsight is the ability of some cortically blind patients to discriminate visual events presented within their field defect. We have examined a fundamental aspect of visual processing, namely the detection of spatial structures presented within the field defect of 10 cortically blind patients. The method outlined is based on the detection of high‐contrast stimuli and is effective in flagging a ‘window of detection’ in the spatial frequency spectrum, should it exist. Here we report on the presence of a narrowly tuned psychophysical spatial channel optimally responding to frequencies less than 4 cycles/° in eight out of 10 patients tested. The two patients who did not show any evidence of blindsight appear to have intact midbrain structures, but have lesions that extend from the occipital cortex to the thalamus. In addition, we have recorded subjective reports of awareness of the visual events in each trial. Detection scores of eight blindsight patients were subsequently subdivided based on the subjective reports of awareness. It appears that the psychophysical spatial channel‐mediating responses in the absence of any awareness of the visual event have a narrower frequency response than those involved when the patients report some awareness of the visual event. The findings are discussed in relation to previous reports on the incidence of blindsight and performance on tasks involving spatial processing.

Psychophysical and pupillometric study of spatial channels of visual processing in blindsight

To date no systematic method has been used for characterising the residual capacity of blindsight subjects that would allow comparison and generalisation across all subjects. The detection of isoluminant gratings of varying spatial and temporal frequencies commends itself for detailed between-subject comparison, and for mapping results onto physiological properties in relation to neuronal circuitry. We report the ability of a blindsight subject (CS) to detect suprathreshold sine-wave gratings over a range of spatial and temporal frequencies using psychophysical techniques. A band-pass spatial channel with an upper cutoff below 3.5 cycles/deg is specified. The data also have been analysed to compare differences between two types of blindsight performances, type I and type II. Spatial gratings were also used to elicit a pupillary grating response, offering an objective method that is free of verbal nuances and response bias, and the resulting band-pass channel can be used both for clinical screening and for prediction and comparisons with psychophysical profiles. Finally, we have compared our results with those reported in studies of a well-known subject, GY, which demonstrate remarkable similarities. Implications are discussed in relation to blindsight research.

Pupil response triggered by the onset of coherent motion

Abstract• Background: Recent studies have shown that transient constrictions of the pupil can be elicited by visual stimuli that do not cause an increment in light flux level on the retina. Such stimuli include achromatic gratings and isoluminant chromatic patterns. • Method: We investigated pupillary responses to the onset of coherent movement generated in a pattern of dots in random motion. Measurements were carried out in normal observers and in a subject with hemianopia caused by damaged primary visual cortex. • Results: The experimental findings show that the onset of coherent motion triggers systematic constrictions of the pupil that cannot be accounted for in terms of a pupil light reflex response. We labelled these constrictions Pupil motion responses (PMRs). Results show that PMRs have large response latencies and on average are of small response amplitudes. The dependence of PMRs on changes in motion parameters such as stimulus speed and direction of motion has been investigated. • Conclusions: The existence of PMRs to the onset of the coherent motion in human vision has been demonstrated. These new findings are discussed in relation to the psychophysical and physiological data on motion perception and the possible pathways involved in the control of the pupil response.