Paul Azzopardi

The responses of neurons in the temporal cortex of primates, and face identification and detection.

The ability of a human observer to detect the presence of a briefly flashed picture of a face can depend on the pictures spatial configuration, that is on whether its features are rearranged (jumbled) or are in their normal configuration. The face-detection effect (FDE) is found under conditions of backward masking, when the presence of a face can be detected with shorter masking intervals when it is in the normal than when in the rearranged configuration. A similar effect is found when the subject is asked to classify the face as rearranged or not — the face-classification effect (FCE). Part of the interest of the FDE and the FCE is that they show how the configuration of a stimulus can be an important factor in the perceptual processing which leads to detection and classification of the stimulus. To analyse these effects we recorded from single neurons in the cortex in the superior temporal sulcus of macaques when they were shown (in a visual fixation task) normal and rearranged faces under backward masking conditions shown in experiments 2 and 3 to produce, with the same apparatus, the FCE, and also to produce comparable effects on the identification of which face was present (called hereafter the face-identification effect), and also of the clarity of the face. We found in experiment 1 that there are some face-selective neurons which respond to faces only, or better, when the features in the faces are in their normal configuration rather than rearranged. We also showed in this experiment that the difference in the response to the normal as compared to the rearranged faces became greater when the masking stimulus was delayed more. Thus, at intermediate delays, there are more neurons active for the normal than for the rearranged face. We therefore propose that the FDE, the FCE, and the face-identification effect arise because the total number of neurons activated by faces in their normal configuration is greater than that activated by rearranged faces, because of the sensitivity of some face-selective neurons to the spatial arrangement of the features. The experiments also show that backward visual masking does produce abrupt termination of the firing of neurons in the temporal cortical visual system, so that the duration of a neuronal response is very short when visual stimuli can just be perceived.

Uneven mapping of magnocellular and parvocellular projections from the lateral geniculate nucleus to the striate cortex in the macaque monkey

Central vision is substantially over represented in the lateral geniculate nucleus (dLGN) and striate cortex. The over representation could be accompanied by a selective expansion of central vision in parvocellular dLGN, in which case the ratio of parvocellular to magnocellular inputs to striate cortex should change with retinal eccentricity. To test this, sample ratios were determined from counts of neurons in dLGN labelled retrogradely with WGA-HRP from striate cortex at the cortical representations of various eccentricities. Parvocellular to magnocellular ratios decreased from a mean of 35:1 at the fovea to 5:1 at 15 degrees eccentricity. Furthermore, they exceeded the ratio of P beta to P alpha ganglion cells in central retina, but not in peripheral retina, showing that the uneven P to M ratio in the LGN does not merely mirror the distribution of ganglion cells in the retina. This provides direct evidence for selective over representation of central vision in parvocellular dLGN.

The role of light scatter in the residual visual sensitivity of patients with complete cerebral hemispherectomy.

Various residual visual capacities have been reported for the phenomenally blind field of hemispherectomized patients, providing evidence for the relative roles of cortical and subcortical pathways in vision. We attempted to characterize these functions by examining the ability of five patients to detect, localize, and discriminate high-contrast flashed, flickering and moving targets. Dependent measures were verbal, manual, and oculomotor responses. As a control for light scatter, intensity thresholds for monocular detection of targets in the hemianopic field were compared with thresholds obtained when using an additional half eyepatch to occlude the blind hemiretina of the tested eye. One unilaterally destriate patient was tested on the same tasks. In photopic conditions, none of the hemispherectomized patients could respond to visual cues in their impaired fields, whereas the destriate patient could detect, discriminate, and point to targets, and appreciate the apparent motion of stimuli across his midline. Under reduced lighting, the threshold luminance required by hemispherectomized patients to detect stimuli presented monocularly was similar to that required for their detection when all visual information was occluded in the blind field, and only available to the visual system indirectly via light scatter. In contrast, the destriate patients monocular threshold in his blind field was substantially lower than that for stimuli directly occluded in the blind field. As we found no range of stimuli which the hemispherectomized patients could detect or discriminate that was not also associated with discriminable scattered light, we conclude that the subcortical pathways which survive hemispherectomy cannot mediate voluntary behavioural responses to visual information in the hemianopic field.

Illusory motion perception in blindsight

Motion detection is typically spared in blindsight, which results from damage to the striate cortex (area V1) of the brain that is sufficient to eliminate conscious visual awareness and severely reduce sensitivity to luminance contrast, especially for high spatial and low temporal frequencies. Here we show that the discrimination of motion direction within cortically blind fields is not attributable to feature tracking (the detection of changes in position or shape), but is due instead to the detection of first-order motion energy (spatiotemporal changes in luminance). The key to this finding was a version of the line motion illusion entailing reverse-phi motion in which opposing motion directions are simultaneously cued by motion energy and changes in stimulus shape. In forced-choice tests, a blindsighted test subject selected the direction cued by shape change when the stimulus was presented in his intact field, but reliably selected the direction cued by motion energy when the same stimulus was presented in his blind field, where relevant position information was either inaccessible or invalid. Motion energy has been characterized as objectless, so reliance on motion energy detection is consistent with impaired access to shape information in blindsight. The dissociation of motion direction by visual field (cortically blind vs. intact) provides evidence that two pathways from the retina to MT/V5 (the cortical area specialized for motion perception) are functionally distinct: the retinogeniculate pathway through V1 is specialized for feature-based motion perception, whereas the retinocollicular pathway, which bypasses V1, is specialized for detecting motion energy.

Models of ganglion cell topography in the retina of macaque monkeys and their application to sensory cortical scaling.

We devised mathematical models of the topography of ganglion cells in the retina of macaque monkeys. The models consisted of a sum-of-three exponentials function fitted to measurements of ganglion cell density made on the nasal horizontal meridian, combined with known anisotropies across the horizontal and vertical meridians by means of elliptic interpolation to provide a full description of their density across the whole of the retinal surface. Integration using standard numerical techniques allowed the number of ganglion cells in arbitrary regions of the retina to be estimated. The topography of actual and effective total ganglion cell populations, and of primate alpha and gamma retinal ganglion cells, was modelled on previously published data. The models were used to test the hypothesis that the retinal projection to the striate cortex in macaque monkeys is peripherally scaled (i.e. merely reflects the eccentricity-dependent variation in density of ganglion cells in the retina) by comparing the cumulative proportion of ganglion cells with the cumulative proportion of cortical area as a function of eccentricity in the visual field. Discrepancies between the two curves indicated that the fovea and immediately surrounding retina are overrepresented in the striate cortex (i.e. there is more cortex per ganglion cell in and near the fovea than in the periphery), and the fact that the discrepancies persisted out to 25-50 degrees of eccentricity showed that the overrepresentation cannot be explained by the lateral displacement of foveal ganglion cells.

Absolute and relative blindsight

The concept of relative blindsight, referring to a difference in conscious awareness between conditions otherwise matched for performance, was introduced by Lau and Passingham (2006) as a way of identifying the neural correlates of consciousness (NCC) in fMRI experiments. By analogy, absolute blindsight refers to a difference between performance and awareness regardless of whether it is possible to match performance across conditions. Here, we address the question of whether relative and absolute blindsight in normal observers can be accounted for by response bias. In our replication of Lau and Passinghams experiment, the relative blindsight effect was abolished when performance was assessed by means of a bias-free 2AFC task or when the criterion for awareness was varied. Furthermore, there was no evidence of either relative or absolute blindsight when both performance and awareness were assessed with bias-free measures derived from confidence ratings using signal detection theory. This suggests that both relative and absolute blindsight in normal observers amount to no more than variations in response bias in the assessment of performance and awareness. Consideration of the properties of psychometric functions reveals a number of ways in which relative and absolute blindsight could arise trivially and elucidates a basis for the distinction between Type 1 and Type 2 blindsight.

Thresholds for detection and awareness of masked facial stimuli.

It has been suggested that perception without awareness can be demonstrated by a dissociation between performance in objective (forced-choice) and subjective (yes-no) tasks, and such dissociations have been reported both for simple stimuli and more complex ones including faces. However, signal detection theory (SDT) indicates that the subjective measures used to assess awareness in such studies can be affected by response bias, which could account for the observed dissociation, and this was confirmed by Balsdon and Azzopardi (2015) using simple visual targets. However, this finding may not apply to all types of stimulus, as the detectability of complex targets such as faces is known to be affected by their configuration as well as by their stimulus energy. We tested this with a comparison of forced-choice and yes-no detection of facial stimuli depicting happy or angry or fearful expressions using a backward masking paradigm, and using SDT methods including correcting for unequal variances in the underlying signal distributions, to measure sensitivity independently of response criterion in 12 normal observers. In 47 out 48 comparisons there was no significant difference between sensitivity (da) in the two tasks: hence, across the range of expressions tested it appears that the configuration of complex stimuli does not enhance detectability independently of awareness. The results imply that, on the basis of psychophysical experiments in normal observers, there is no reason to postulate that performance and awareness are mediated by separate processes.