Lawrence Weiskrantz

The effect of prior learning on subsequent retention in amnesic patients

Abstract Amnesic patients can demonstrate normal retention of verbal material using a cued recall retrieval technique. Five experiments are reported in which the properties and limits of cued recall were explored. Certain qualitative differences in the performance of amnesic subjects compared with control subjects were obtained. First, the phenomenon of differential efficacy of cued recall in amnesic subjects has been replicated. Second, restricting the number of response alternatives is more effective in amnesic subjects than normal subjects. Third, some evidence for enhanced effects of prior learning on retention has been shown in the amnesic subjects. These findings are incompatible with an interpretation in terms of a failure of consolidation, but are compatible with one that attaches importance to interference phenomena in the amnesic syndrome.

Non-conscious recognition of affect in the absence of striate cortex

Functional neuroimaging experiments have shown that recognition of emotional expressions does not depend on awareness of visual stimuli and that unseen fear stimuli can activate the amygdala via a colliculopulvinar pathway. Perception of emotional expressions in the absence of awareness in normal subjects has some similarities with the unconscious recognition of visual stimuli which is well documented in patients with striate cortex lesions (blindsight). Presumably in these patients residual vision engages alternative extra-striate routes such as the superior colliculus and pulvinar. Against this background, we conjectured that a blindsight subject (GY) might recognize facial expressions presented in his blind field. The present study now provides direct evidence for this claim.

Conditioning in amnesic patients

Abstract An early clinical observation by Claparede suggests that classical conditioning occured in an amnesic patient even though the subject did not acknowledge memory for the conditioning as such. This suggestion is confirmed in two amnesic subjects (one alcoholic Korsakoff patient and one probable encephalitic patient) using the conditioned eye blink. Both subjects showed evidence of conditioning with retention across intervals of 10 min and 24 hr, and with both there was a striking dissociation between their objective performances and their commentaries. Conditioning apparently should be added to the growing list of examples of adequate long-term retention in amnesic subjects, the variety of which makes it difficult to interpret the deficit in terms of a failure of consolidation of particular types of material or of an insensitivity to particular encoding requirements. The theoretical basis of the dissociation between performance and commentary would appear to warrant further development.

Further analysis of the prior learning effect in amnesic patients

Abstract Previous work using cued recall techniques has demonstrated that the learning of a prior list differentially interferes with learning of a subsequent list by amnesic subjects. Three experiments are reported here in which an interference hypothesis is put to the test. One question is whether interference effects appear immediately during testing of the second task. The effects of eliminating response competition are studied, as well as the importance of response availability. An equivalent increase in intrusive interference effects was obtained in amnesic and control groups alike on the first trial of the second task. Differences between the groups emerged only as additional trials were given on the second task. No differential interference effects emerged as a function of response availability. Theories of amnesia based on an inability to constrain competing responses are not supported by the findings, and therefore at least some forms of an interference hypothesis will not account for the results, which are likely to reflect some other form of an underlying retrieval deficit.

Attention without awareness in blindsight

The act of attending has frequently been equated with visual awareness. We examined this relationship in ‘blindsight’: a condition in which the latter is absent or diminished as a result of damage to the primary visual cortex. Spatially selective visual attention is demonstrated when information that stimuli are likely to appear at a specific location enhances the speed or accuracy of detection of stimuli subsequently presented at that location. In a blindsight subject, we showed that attention can confer an advantage in processing stimuli presented at an attended location, without those stimuli entering consciousness. Attention could be directed both by symbolic cues in the subjects spared field of vision or cues presented in his blind field. Cues in his blind field were even effective in directing his attention to a second location remote from that at which the cue was presented. These indirect cues were effective whether or not they themselves elicited non–visual awareness. We concluded that the spatial selection of information by an attentional mechanism and its entry into conscious experience cannot be one and the same process.

Unseen facial and bodily expressions trigger fast emotional reactions

The spontaneous tendency to synchronize our facial expressions with those of others is often termed emotional contagion. It is unclear, however, whether emotional contagion depends on visual awareness of the eliciting stimulus and which processes underlie the unfolding of expressive reactions in the observer. It has been suggested either that emotional contagion is driven by motor imitation (i.e., mimicry), or that it is one observable aspect of the emotional state arising when we see the corresponding emotion in others. Emotional contagion reactions to different classes of consciously seen and “unseen” stimuli were compared by presenting pictures of facial or bodily expressions either to the intact or blind visual field of two patients with unilateral destruction of the visual cortex and ensuing phenomenal blindness. Facial reactions were recorded using electromyography, and arousal responses were measured with pupil dilatation. Passive exposure to unseen expressions evoked faster facial reactions and higher arousal compared with seen stimuli, therefore indicating that emotional contagion occurs also when the triggering stimulus cannot be consciously perceived because of cortical blindness. Furthermore, stimuli that are very different in their visual characteristics, such as facial and bodily gestures, induced highly similar expressive responses. This shows that the patients did not simply imitate the motor pattern observed in the stimuli, but resonated to their affective meaning. Emotional contagion thus represents an instance of truly affective reactions that may be mediated by visual pathways of old evolutionary origin bypassing cortical vision while still providing a cornerstone for emotion communication and affect sharing.

A STUDY OF LEARNING AND RETENTION IN AMNESIC PATIENTS

Abstract The decay characteristics of the forgetting curve in amnesic subjects was examined as a function of the degree of original learning; the effect of number of learning trials and interval on retention was systematically measured. Six patients with amnesic syndromes and five control subjects were tested. Retention measured by free recall, and two forms of recognition were compared. The amnesic group was impaired relative to the control group under all conditions. Insufficient learning was achieved by the amnesic group for comparison of rate of forgetting in the two groups. Certain qualitative differences between the amnesic subjects and the control group emerged. Relative to the control group the amnesic group apparently showed a greater deficit in retention measured by recognition than recall. False positive errors were recorded and evidence of proactive interference in the amnesic group is presented.

Varieties of Residual Experience

The apparent discrepancy between the results of animal studies and clinical findings on the effects of occipital lobe damage became even greater with the results of animal experiments over the past 20 years in which subjects showed significant residual vision. But there is now evidence that the human subject can also show considerable residual capacity. It appears to be extremely helpful to use forced choice discrimination methods and specific training, as with animals, rather than to depend on verbal commentary by the human subject, who may be unaware of his discriminative capacity. Where man and monkey can be compared, it would appear that they are not qualitatively different, and that residual capacity is biased for detection and localisation rather than identification. Preliminary evidence is presented on the incidence of “blind-sight” and related aspects of residual function, including examination of different dimensions such as form, detection, movement, orientation, and spatial localisation within field defects. Some of these dimensions appear to be dissociable, but each also seems capable of disconnexion from the subjects own commentary and acknowledgement.

A Perimetric Study of Visual Field Defects in Monkeys

The visual fields of rhesus monkeys have been studied perimetrically before and after removal of parts of the striate cortex. The operations produced visual field defects of the expected size, shape, and position but an animals ability to respond to a flash of light which appeared in the defective part of the field was diminished rather than abolished. It is suggested that this residual ability enables an animal to detect changes in illumination, which might be cues to other visual events. A study of fixation indicates that the animals probably do not recognize or respond to objects when they lie within the impaired region of the visual field. The results are compared with those found in earlier studies of simian and human subjects.

EVIDENCE FOR DISSOCIATION OF IMPAIRMENT ON AUDITORY DISCRIMINATION AND DELAYED RESPONSE FOLLOWING LATERAL FRONTAL LESIONS IN MONKEYS

Impairments on both delayed-response and auditory-discrimination tasks have been reported to follow lateral frontal cortical lesions in monkeys. The present study is primarily concerned with the relationship of these two deficits. Three monkeys received ablations of sulcus principalis; three received ablations of lateral frontal cortex excluding sulcus principalis. Acquisition and retention of auditory-discrimination, delayed-response, and visual-discrimination tasks by the two groups were compared. Sulcus principalis lesions produced greater impairment on the delayed-response tasks. The lesions sparing sulcus principalis produced greater impairment on the auditory-discrimination tasks. The performance of the groups did not differ on the visual-discrimination tasks. These results suggest that the delayed-response deficit and the deficit in auditory discrimination that follow large frontal lesions may be dissociated by smaller frontal lesions. The results confirm that the focus for impairment on tests of the delayed-response type is in the region of sulcus principalis. The experimental data do not indicate the focus (if any) for the auditory-discrimination deficit, nor do they permit the definition of the nature of this deficit including its modality specificity.