Speculations on the neural basis of islands of blindsight.

Blindsight, residual visual function in the absence of conscious awareness, can sometimes be found within the scotomas of patients with lesions to primary visual cortex. However, cases in which blindsight is distributed across large regions of a scotoma are extremely rare. In contrast, blindsight is relatively frequent within small islands of residual visual function. We review the evidence for the existence of these islands. We argue that blindsight is likely to depend on vestiges of geniculostriate function, and that in humans the secondary retinotectal pathway has little functional utility in the absence of geniculostriate support. To account for the frequency of blindsight within residual islands of function, we speculate that patients may be unaware of such islands precisely because they are islands, which are isolated from the integrated network of neural activity that represents visual space. The relationship of blindsight to the hemispatial neglect is considered in this context.

Hierarchical organization of the human auditory cortex revealed by functional magnetic resonance imaging.

The concept of hierarchical processing--that the sensory world is broken down into basic features later integrated into more complex stimulus preferences--originated from investigations of the visual cortex. Recent studies of the auditory cortex in nonhuman primates revealed a comparable architecture, in which core areas, receiving direct input from the thalamus, in turn, provide input to a surrounding belt. Here functional magnetic resonance imaging (fMRI) shows that the human auditory cortex displays a similar hierarchical organization: pure tones (PTs) activate primarily the core, whereas belt areas prefer complex sounds, such as narrow-band noise bursts.

A positron emission tomographic study of auditory localization in the congenitally blind.

We have used positron emission tomography (PET) to measure regional cerebral blood flow (rCBF) in sighted and congenitally blind subjects performing auditory localization tasks. During scanning, the spectral and binaural cues of localized sound were reproduced by a sound system and delivered via headphones. During tasks that required auditory localization both the sighted and blind subjects strongly activated posterior parietal areas. In addition, the blind subjects activated association areas in the right occipital cortex, the foci of which were similar to areas previously identified in visual location and motion detection experiments in sighted subjects. The blind subjects, therefore, demonstrated visual to auditory cross-modal plasticity with auditory localization activating occipital association areas originally intended for dorsal-stream visual processing. To determine the functional connectivity of pre-selected brain regions in primary and non-primary auditory and posterior parietal cortex in the two cohorts, we performed an inter-regional correlation analysis on the rCBF data set. During auditory localization in the blind subjects, rCBF activity in the right posterior parietal cortex was positively correlated with that in the right occipital region, whereas in sighted subjects correlations were generally negative. There were no significant positive occipital correlations in either cohort when reference regions in temporal or left parietal cortex were chosen. This indicates that in congenitally blind subjects the right occipital cortex participates in a functional network for auditory localization and that occipital activity is more likely to arise from connections with posterior parietal cortex.

A PET study of human auditory spatial processing.

To learn more about human auditory spatial processing, we used positron emission tomography (PET) to measure regional cerebral blood flow in human volunteers engaged in sound localization tasks. Spectral and binaural cues of localized sound were reproduced by a sound system and delivered via headphones. During localization tasks, subjects activated inferior parietal lobules (IPL) bilaterally. In a second experiment, matched in design to the first, subjects made non-spatial auditory discriminations based on frequency, activating the IPL bilaterally with left hemispheric predominance. A between-study comparison revealed that the right IPL was significantly more activated during the sound localization task compared with the feature discrimination task, suggesting a preferential role for the right IPL in auditory spatial processing.

Variability of residual vision in hemianopic subjects.

Using stabilized visual field mapping techniques, seven hemianopic subjects were extensively investigated for residual visual abilities. Isolated islands of detection abilities were demonstrated by four of these subjects. Additional abilities demonstrated within these islands included saccadic and verbal localization, wavelength discrimination, form discrimination, and motion detection. These abilities were also accompanied by low-confidence ratings, and thus have the character of blindsight. It is noteworthy that different subjects demonstrated different abilities at different visual field locations, underscoring the between and within subject variability often observed with blindsight. Furthermore, magnetic resonance images obtained for each subject demonstrated variable sparing of occipital cortex. Such cortical sparing, in conjunction with the behavioral variability, supports the notion that some instances of blindsight are mediated by remnants of the primary visual pathway.

Tonotopy in human auditory cortex examined with functional magnetic resonance imaging.

Tonotopic organization within the human auditory cortex was investigated with functional magnetic resonance imaging (fMRI) using the blood oxygenation level-dependent (BOLD) contrast mechanism. Single-frequency pulsed tones were alternated with no-tone conditions to elicit stimulus-specific functional activity. Differential frequency-specific activity was imaged within the auditory cortex Activations for high-frequency tones were located more posteriorly and medially than those for low-frequency tones. Such a pattern is consistent with descriptions of tonotopic organization suggested by other nonneuroimaging methodologies used with human and nonhuman primates. Furthermore, these results demonstrate that fMRI can be used to reliably investigate functional organization of the human auditory cortex. Hum. Brain Mapping 4:18-25, 1997. (c) 1997 Wiley-Liss, Inc.

Islands of residual vision in hemianopic patients.

Following damage to primary visual cortex, some patients our initial findings. The data reveal a patchy distribution of redemonstrate a limited ability to respond to stimuli they do not sidual visual abilities in the absence of acknowledged awareness. acknowledge seeing. This residual vision, which has been referred to as "blindsight," has been attributed to secondary visual pathways. We previously reported an isolated island of blindsight in one patient and argued it was a likely consequence of cortical sparing in V1. We now report an extension of our initial findings. The data reveal a patchy distribution of residual visual abilities in the absence of acknowledged awareness. Variable patterns of cortical sparing appear to be the most parsimonious way to account for this outcome, suggesting that blindsight is generally mediated by the primary visual pathway.

Residual vision with awareness in the field contralateral to a partial or complete functional hemispherectomy.

Two patients with unilateral disconnection or removal of the entire occipital lobe were tested for residual vision in their blind field. Using image stabilization to eliminate eye motion artifacts, the central portion of each subject's visual field was tested, beginning 1 degree from fixation and extending outward to 13.5 degrees. A narrow zone of residual vision was identified along the retinal vertical meridian of each patient. The lateral edge of this zone was generally within 3.5 degrees of the vertical meridian, though extended farther outward (but not beyond 6 degrees) at one field location for each subject. In one patient, it was present in both superior and inferior quadrants; in the other, it was confined to the superior quadrant. Within their zones of residual vision, both patients could detect stimuli and perform simple shape discriminations, but could not name complex line drawings. The patients were aware of their vision within this zone. No residual vision, with or without awareness, was found in areas tested outside these zones. Given the complete absence of visual cortex contralateral to the observed residual vision, alternate structures must be mediating these abilities.

Collaboration between the hemispheres of a callosotomy patient. Emerging right hemisphere speech and the left hemisphere interpreter.

Split brain patients who are initially unable to produce speech in their right hemispheres sometimes develop the ability to do so. Patient J.W., the subject of this report, is such a patient. At the time of his callosotomy, J.W. had a language dominant left hemisphere; his right hemisphere could understand both spoken and written language, but he was unable to speak. Fourteen years after his surgery, we found that J.W. was capable of naming approximately 25% of the stimuli presented to his left visual field (LVF). Now, 1 year later, we find that he can name about 60% of such stimuli. This late-developing speech ability appears to be consequence of long-term neural plasticity. However, the subject's extended verbal responses to LVF stimuli seem to result from a collaboration between the hemispheres and to involve the left hemisphere interpreter.

Nasotemporal overlap at the retinal vertical meridian: investigations with a callosotomy patient.

To evaluate nasotemporal overlap at the retinal vertical midline, we asked a callosotomy patient to compare the orientation of square wave gratings that were presented in his opposing visual fields. The gratings were horizontally or vertically oriented and had spatial frequencies ranging from 1 to 8 cycles per degree (cpd). Retinal stabilization ensured the gratings remained properly lateralized during sustained presentations. In accord with previous investigations, when the gratings were presented for only 200 msec, or their medial edges were 2 from the vertical meridian, performance was generally at chance. However, when presentations lasted 2 sec and the medial edges of the gratings were 1 from the vertical meridian, above chance performance was obtained. Accuracy rates were highest with 2 and 4 cpd gratings, and dropped at 1 and 8 cpd. Unexpectedly, the subject performed significantly better when the gratings were offset vertically from each other than when both were displayed on the horizontal meridian. Since this improvement did not occur when gratings were presented horizontally aligned above or below the horizontal meridian, it must be distributed to the relative offset between the gratings. The data suggest a narrow zone of nasotemporal overlap at the retinal vertical midline where very limited visual information is encoded by crossed temporal and uncrossed nasal retinal ganglion cells. An experiment in which only one grating in a pair was close to the vertical meridian indicates that this zone may be more pronounced in the nasal hemiretina.

Extrageniculostriate vision in humans: investigations with hemispherectomy patients.

Residual vision was assessed in the blind hemifield of one hemispherectomized and one partially hemispherectomized patient, using an interval two alternative forced choice detection task. Fixation instabilities were controlled by retinal stabilization. In both patients, residual vision was found in the hemianopic field close to the vertical meridian. This residual vision is largely confined to a band not wider the 3 degrees, but there is a local region in each patient where it extends more than 3 degrees from the meridian. However, more than 6 degrees from the vertical meridian we found no indication of residual function in either patient. Within the band of spared vision, subjects are aware of stimuli and can perform simple shape discriminations. Visual acuity profiles argue against an explanation based on eccentric fixation. Explanations based on the retino-tectal pathway or on retinal naso-temporal overlap are possible.

The emergence of the capacity to name left visual field stimuli in a callosotomy patient: implications for functional plasticity.

Callosotomized patient J.W. has a well-documented history of right hemisphere language abilities, including an auditory and visual lexical-semantic system with limited phonology and syntax. However, J.W. has not previously exhibited the ability to name stimuli presented to the left visual field (LVF). We report the emergence of this ability. Experiments were conducted in which pictures and text were presented to the subject's LVF using retinal stabilization techniques to ensure lateralization. J.W. was able to correctly name approximately one-quarter of these stimuli under a variety of presentation conditions. The newly developed ability to respond verbally to complex LVF stimuli can be the result of (1) enhanced inter-hemispheric transfer of information via sub-cortical pathways, (2) sophisticated cross cueing strategies, or (3) control of motor speech in the right hemisphere. Although it appears that the first two mechanisms make a contribution to J.W.'s LVF naming performance, accuracy for unpredictable stimulus sets and the error patterns require acknowledgement that control of motor speech is now available to the right hemisphere.

Are microsaccades responsible for the gap effect?

Extinguishing a fixation point shortly before, or concurrently with, the onset of a peripheral visual target reduces the latency of saccades to that target. Saslow (1967) hypothesized that this gap effect might occur because fixation point offsets reduce the incidence of corrective microsaccades with an associated saccadic refractory period. In the present study, a robust gap effect was obtained. However, using a Purkinje image eyetracker with 1 arcmin of resolution, we found that fixation point offsets had no effect on the occurrence of microsaccades and that the occurrence of microsaccades had no impact on the magnitude of gap effect. Microsaccades therefore do not appear to play any part in the production of the gap effect.

Cerebral and callosal organisation in a right hemisphere dominant "split brain" patient.

Patients described in previous reports who have undergone corpus callostomy for control of seizures have been left hemisphere dominant for language. To determine the hemispheric localisation (and possible coexistence) of language and traditional right hemisphere skills in reversed dominance, the first right hemisphere dominant corpus callostomy patient was studied. Localisation of callosal functions was also investigated, as MRI showed 1.5 cm of spared callosal body. The patient, KO, a 15 year old girl with familial left handedness, underwent two stage callosotomy in 1988. Lateralised visually presented stimuli requiring same or different comparisons between visual fields showed chance performance. Oral naming and reading showed better performance by the right hemisphere than the left, whereas both hemispheres were proficient in auditory comprehension. Active voice syntax was above chance only in the right hemisphere. Face recognition was significantly better in the right hemisphere than in the left. Tasks requiring tactile comparisons between hands showed above chance performance except in the instance in which the non-dominant right hand was stimulated first in a point localisation task between hands. This case showed hemispheric coexistence of language and traditional right hemispheric skills in a corpus callosotomy patient with reversed language dominance. Tactile transfer was localised to the mid-posterior callosal body.

Residual vision in a scotoma: implications for blindsight.

Blindsight, the ability of some blind patients to describe attributes of stimuli they have no conscious awareness of seeing, has been attributed to a secondary (retinotectal) visual pathway. However, it has also been proposed that blindsight could be due to residual function within the primary (geniculostriate) visual pathway. Data have now been obtained that support the second alternative. With an image stabilizer ensuring the accurate retinal placement of stimuli, dense visual field mapping was carried out with a hemianopic patient. This perimetry revealed, embedded in the patient's scotoma, an isolated 1-degree island of residual vision that was not disclosed by conventional perimetric methods. Stimuli presented to this island could be detected and discriminated, although the subject reported he did not see them. The existence of this island of vision implies a corresponding island of functioning cortex within the patient's lesion. Other instances of blindsight may be mediated by similar islands of functioning cortex.