Petra Stoerig

The neurobiology of blindsight

Some patients can respond to visual stimuli presented within their clinically absolute visual field defects that have been caused by partial destruction of striate cortex. This puzzling phenomenon of looking, pointing, detecting and discriminating without seeing has been called blindsight, and has fascinated philosophers and neuroscientists alike as a spotlight on the nature of unconscious or covert awareness, and the means it provides of studying the visual information carried by pathways other than the major route through the striate cortex.

Neural correlates of religious experience.

The commonsense view of religious experience is that it is a preconceptual, immediate affective event. Work in philosophy and psychology, however, suggest that religious experience is an attributional cognitive phenomenon. Here the neural correlates of a religious experience are investigated using functional neuroimaging. During religious recitation, self‐identified religious subjects activated a frontal–parietal circuit, composed of the dorsolateral prefrontal, dorsomedial frontal and medial parietal cortex. Prior studies indicate that these areas play a profound role in sustaining reflexive evaluation of thought. Thus, religious experience may be a cognitive process which, nonetheless, feels immediate.

ERP—Correlates of response selection in a response conflict paradigm

Neuroimaging and electrophysiological studies suggest that the anterior cingulate cortex (ACC) is involved in the cognitive control of response related action. A frontocentral negative ERP-component, the N2, which probably originates from the ACC, is usually enhanced in conflict-trials that demand an unexpected response. We here used stepped adjustment of response expectation in a response-cueing task, and measured how the N2 varied with global and local cue validity. Results showed that, irrespective of the current cue validity, response times, error rates, and the frontocentral components P2, N2 and P3 increased in unexpected trials. Nevertheless, a N2 was also seen in expected trials, and its latency correlated positively with reaction times, indicating that this potential does not express response conflict only. In line with roles suggested for the ACC, we here propose that the N2 is related to the process of response selection which influences subsequent processing stages reflected in the P3. Unexpected revisions of response programs enhance and delay the N2.

Projection patterns of surviving neurons in the dorsal lateral geniculate nucleus following discrete lesions of striate cortex: implications for residual vision.

SummaryIn four monkeys with long-standing partial ablation of the striate cortex pellets of horseradish peroxidase were placed in either the striate cortex immediately adjacent to the ablation, or in the extrastriate cortex of the ventral prelunate gyrus, i.e. in visual area V4. We examined the dorsal lateral geniculate nucleus to see whether surviving neurons, within the region that shows retrograde degeneration as a result of the cortical lesion, project to remaining striate cortex and/or to extrastriate cortex. Neurons labelled from extrastriate cortex were found throughout the degenerated region, whereas neurons labelled from striate cortex were confined to the border between the normal and degenerated region of the nucleus. This shows that isolated neurons found within the degenerated region survive striate cortex damage because they project to an extrastriate visual area, and not because their terminals depart from the otherwise strict topographic representation of the lateral geniculate nucleus on to striate cortex.

Signal detection analysis of residual vision in a field defect due to a post-geniculate lesion

A patients ability to discriminate between blank trials and small visual targets presented tachistoscopically within his blind hemifield was tested at five positions within the scotoma. For each position a Receiver-Operating-Characteristic curve (ROC) was determined. Results show the patients discrimination between the target and no-target conditions to be better than expected by guessing at three positions along the horizontal meridian, whereas in the natural Blind Spot within the scotoma no significant discrimination is found. Thus, light scatter produced by the visual target cannot explain the observed residual vision. At least three anatomical pathways could be responsible for the discrimination: the retino-collicular projection, the retino-geniculo-striatal projection, and the retino-geniculo-extrastriatal projection.

Varieties of vision: from blind responses to conscious recognition

Lesions in consecutive parts of the visual system cause visual deficits that spare increasingly complex residual functions. Patients with lesions up to and including primary visual cortex can show neuroendocrine, reflexive, implicit and forced-choice responses to visual stimulation but no conscious vision. In contrast, patients with lesions in higher visual cortical areas have conscious vision. Its lowest level is that of phenomenal vision, followed by object vision and recognition. These levels are dissociable. They require the integrity of different parts of the system.

Low-level phenomenal vision despite unilateral destruction of primary visual cortex.

GY, an extensively studied human hemianope, is aware of salient visual events in his cortically blind field but does not call this vision. To learn whether he has low-level conscious visual sensations or whether instead he has gained conscious knowledge about, or access to, visual information that does not produce a conscious phenomenal sensation, we attempted to image process a stimulus s presented to the impaired field so that when the transformed stimulus T(s) was presented to the normal hemifield it would cause a sensation similar to that caused by s in the impaired field. While degradation of contrast, spatio-temporal filtering, contrast reversal, and addition of smear and random blobs all failed to match the response to a flashed bar s(f), moving textures of low contrast were accepted to match the response to a moving contrast-defined bar, s(m). Orientation and motion direction discrimination of the perceptually matched stimuli [s(m) and T(s(m))] was closely similar. We suggest that the existence of a satisfactory match indicates that GY has phenomenal vision.

Transneuronal retrograde degeneration of retinal ganglion cells and optic tract in hemianopic monkeys and humans

Transneuronal retrograde degeneration of retinal ganglion cells after removal of primary visual cortex (area V1) is well established by quantitative neurohistological analysis of the ganglion cell layer in monkeys, but remains controversial in human patients. Therefore, we first histologically examined retinal degeneration in sectioned archived retinae of 26 macaque monkeys with unilateral V1 ablation and post-surgical survival times ranging from 3 months to 14.3 years. In addition, the cross-sectional area of the optic tract was measured in archived coronal histological sections of the brain of every hemianopic monkey and in sections from 10 control monkeys with non-visual bilateral cortical lesions. The ratios of nasal and temporal retinal ganglion cell counts in the contralesional eye and ipsi/contralateral optic tract areas were calculated and compared. They show that the decline was initially more pronounced for the optic tract, slackened after 3 years post-lesion and was steeper for the ganglion cells thereafter. Nevertheless, both measures were highly correlated. Second, we calculated ratios from structural magnetic resonance images to see whether the optic tracts of four human hemianopes would show similar evidence of transneuronal degeneration of their ipsilesional optic tract. The results were consistent with extensive and time-dependent degeneration of the retinal ganglion cell layer. The measures of the optic tracts provide evidence for comparable transneuronal retinal ganglion cell degeneration in both primate species and show that structural magnetic resonance image can both reveal and assess it.

Change blindness and time to consciousness.

Detection of changes in a visual scene can be substantially delayed when the original and the modified image are separated by a brief screen flicker. We used this phenomenon of ‘change blindness’ to find when the brain detects the mismatch in relation to when the observer reports it, and whether changes in identity and position are processed similarly. Event‐related brain potentials (ERPs) recorded while the subjects searched for the change in alternating series of images showed that the epoch during which they indicated detection was characterized by a marked positivity from 300 to 700u2003ms. Analysis of data from image presentations preceding the subjects response revealed a similar but smaller ERP positivity one (identity) or even two (position) epochs before detection. As each epoch lasted 1500u2003ms, the brain may register a change as early as 3000u2003ms before the observer.

Seeing ‘Where’ through the Ears: Effects of Learning-by-Doing and Long-Term Sensory Deprivation on Localization Based on Image-to-Sound Substitution

Background Sensory substitution devices for the blind translate inaccessible visual information into a format that intact sensory pathways can process. We here tested image-to-sound conversion-based localization of visual stimuli (LEDs and objects) in 13 blindfolded participants. Methods and Findings Subjects were assigned to different roles as a function of two variables: visual deprivation (blindfolded continuously (Bc) for 24 hours per day for 21 days; blindfolded for the tests only (Bt)) and system use (system not used (Sn); system used for tests only (St); system used continuously for 21 days (Sc)). The effect of learning-by-doing was assessed by comparing the performance of eight subjects (BtSt) who only used the mobile substitution device for the tests, to that of three subjects who, in addition, practiced with it for four hours daily in their normal life (BtSc and BcSc); two subjects who did not use the device at all (BtSn and BcSn) allowed assessment of its use in the tasks we employed. The impact of long-term sensory deprivation was investigated by blindfolding three of those participants throughout the three week-long experiment (BcSn, BcSn/c, and BcSc); the other ten subjects were only blindfolded during the tests (BtSn, BtSc, and the eight BtSt subjects). Expectedly, the two subjects who never used the substitution device, while fast in finding the targets, had chance accuracy, whereas subjects who used the device were markedly slower, but showed much better accuracy which improved significantly across our four testing sessions. The three subjects who freely used the device daily as well as during tests were faster and more accurate than those who used it during tests only; however, long-term blindfolding did not notably influence performance. Conclusions Together, the results demonstrate that the device allowed blindfolded subjects to increasingly know where something was by listening, and indicate that practice in naturalistic conditions effectively improved “visual” localization performance.