Spatial channels of visual processing in cortical blindness

Abstract

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/ degrees 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.

Unconscious fear influences emotional awareness of faces and voices

“…A previously undescribed contribution from the present study is that these two structures are sensitive to the congruence between a seen and an unseen fear expression. Superior colliculus-pulvinar-based residual abilities of hemianopic patients are within a limited range of spatial frequencies (16), which is also sufficient for recognition of facial expressions in normal viewers (14,29).…”

“…But facial expressions can still be recognized when stimuli are much degraded. For example, the facial expression in images with spatial frequencies Ͻ8 cycles can still be recognized almost faultlessly (14,15), and it is interesting to note that this filtering is within the range of spatial frequencies available to the majority of cortically blind patients (16).…”

“…Furthermore, recent explorations of this phenomenon have suggested that the representation of fear in faces is carried in the low spatial frequency component of face stimuli, a component likely to be mediated via a subcortical pathway (14,15). It is perhaps no coincidence that the residual visual abilities of cortically blind subjects are confined to that range (16).…”

Non-conscious recognition of emotional body language

“…This raises the question addressed in the present study of whether the comparatively large-scale features of whole body expressions of emotion can be processed by a subcortical visual route. A major function of this route is to sustain rapid orientation and detection of potentially dangerous signals based on a type of coarse visual analysis as can be performed by the superior colliculus [15]. The possibility that there exists a nonconscious and non-lateral geniculate nucleus-based recognition of emotional bodies is important for understanding the functional contribution of this route in human social communication.…”

Cortico-subcortical visual, somatosensory, and motor activations for perceiving dynamic whole-body emotional expressions with and without striate cortex (V1)

“…Other extrageniculostriate visual routes likely have a much more narrowly specified function, as indicated by their sensitivity to a limited range of spatial frequencies (2), spectral components (3), or motion parameters (4). However, we do not yet have a clear understanding of how these different extrageniculostriate pathways and the visual attributes they process match.…”