Biomedical subjects

L L Baylis

Publications and source records attributed to L L Baylis.

7 recordsLinked to original sources

PubMed2001

Visually misguided reaching in Balint's syndrome.

A patient with bilateral parietal damage leading to Balint's Syndrome was tested on his ability to reach to, and to describe the locations of visual targets. RM was better at reaching to targets than he was at describing the locations of the same targets. Moreover, he was better at reaching to targets when he could not see them, compared to when he was reaching with visual guidance. In a final experiment, we found that RM showed strong inhibition of responses to non-target items, even though he had a poor representation of their location in depth. As a result of intact inhibition and impaired depth representation, he ignored both target and non-target items in a given direction. These results suggest that in RM a disturbed visual representation of space disrupts an otherwise relatively intact reaching control system.

Ataxia↗

PubMed1997

Deficit in figure-ground segmentation following closed head injury.

Patient CB showed a severe impairment in figure-ground segmentation following a closed head injury. Unlike normal subjects, CB was unable to parse smaller and brighter parts of stimuli as figure. Moreover, she did not show the normal effect that symmetrical regions are seen as figure, although she was able to make overt judgments of symmetry. Since she was able to attend normally to isolated objects, CB demonstrates a dissociation between figure ground segmentation and subsequent processes of attention. Despite her severe impairment in figure-ground segmentation, CB showed normal 'parallel' single feature visual search. This suggests that figure-ground segmentation is dissociable from 'preattentive' processes such as visual search.

Cerebral Cortex↗

PubMed1995

Afferent connections of the caudolateral orbitofrontal cortex taste area of the primate.

A cortical taste region has recently been identified in the caudolateral orbitofrontal cortex of the macaque. The afferents to this region were investigated by means of retrograde tracing, using six injections of wheatgerm-conjugated horseradish peroxidase. The area of taste cortex was first identified physiologically in all the monkeys used in this anatomical study. The four injections into the middle and posterior part of this region resulted in large numbers of labelled cell bodies in the insular-opercular primary taste cortex. Following the two more anterior injections, label was found predominantly in the caudal part of the cardolateral orbitofrontal cortex itself. None of the injections resulted in labelled cells in the gustatory thalamic nucleus ventralis posterior medialis, pars parvocellularis, although all injections resulted in label of the mediodorsal nucleus of the thalamus. Afferents were also seen from more anterior parts of the orbitofrontal taste cortex, which may represent backprojections from subsequent taste areas. These results suggest that the caudolateral orbitofrontal cortex contains a higher-order taste cortex.

Amygdala↗

PubMed1994

Reading of letters and words in a patient with Balint's syndrome.

A patient with bilateral parietal damage, and Balint's syndrome, named visual letters. These were presented individually or within four-letter strings. Solitary letters were identified very accurately. In the case of strings, more letters were correctly reported for words than for nonwords, and more for pronounceable than for unpronounceable nonwords. When required to read words as a whole, performance was better than predicted by letter-reports. These results extend the object-based limitation apparent in Balint's syndrome to the case of reading. The component letters of a string benefit when they form a familiar global object, rather than requiring representation as multiple separate objects. The patient occasionally made homophonic errors when listing the letters in a visual word. This suggests an attempt to bypass visual simultanagnosia by treating the string as a single object, deriving a holistic phonological code for it, and then decomposing this into component letters via spelling rules.

Agnosia↗

PubMed1994

Gustatory, olfactory, and visual convergence within the primate orbitofrontal cortex.

Behavioral and perceptual responses to food depend on a convergence between gustatory, olfactory, and visual information. In previous studies a secondary cortical taste area has been found in the caudolateral orbitofrontal cortex of the primate. Furthermore, neurons with olfactory responses have been recorded in a more medial part of the orbitofrontal cortex, and visual inputs have been shown to influence neurons in an intermediate region. These studies suggest that the orbitofrontal cortex may act as a region for convergence of multiple sensory modalities including chemosensation. In the present study neurons throughout the caudal two-thirds of the orbitofrontal cortex of the macaque were tested to gustatory, to olfactory, and to visual stimulation to investigate whether convergence occurs. Neurons in this region of cortex were found that responded to stimulation of the taste, olfactory, or visual system. In addition, some neurons were found with bimodal responses, responding for example to both taste and olfactory, or taste and visual stimuli. Since these multimodal neurons were found in very close proximity to unimodal neurons, and the unimodal sensory neurons were intermingled, it is possible that the orbitofrontal cortex represents the first cortical area of convergence for these three modalities in primates.

Acoustic Stimulation↗

PubMed1991

Amygdalectomy and ventromedial prefrontal ablation produce similar deficits in food choice and in simple object discrimination learning for an unseen reward.

Cynomolgus monkeys (Macaca fascicularis) with either bilateral amygdalectomy or bilateral ventromedial prefrontal cortical ablations showed abnormal choices between apple, lemon, olive, and meat. Not only did they choose meat or olive more often than normal animals, but also their choices were less consistent from trial to trial than the normal animals' choices were. The same animals were subsequently tested for their ability to learn 2-choice simultaneous visual discriminations between objects which they could suck. The positive object yielded fruit juice, which entered the mouth directly without being seen. Both groups of animals with lesions were severely impaired in this discrimination learning task. The reason why amygdalectomy has little effect on simple object discrimination learning in the Wisconsin General Test Apparatus, we suggest, is that the animal can there associate the visual discriminanda with the visual properties of the food reward, a mechanism which is not available when the reward is unseen. These results add to existing evidence of a close functional relationship between the amygdala and the ventromedial prefrontal cortex, and they support the proposal, derived from previous work, that the amygdala is important for associating visual stimuli with the incentive value of reinforcers.

Amygdala↗

PubMed1991

Responses of neurons in the primate taste cortex to glutamate.

In order to investigate the neural encoding of glutamate in the primate, recordings were made from 190 taste responsive neurons in the primary taste cortex and adjoining orbitofrontal cortex taste area in macaques. Single neurons were found that were tuned to respond best to glutamate (umami taste), just as other cells were found with best responses to glucose (sweet), sodium chloride (salty), HCl (sour), and quinine HCl (bitter). Across the population of neurons, the responsiveness to glutamate was poorly correlated with the responsiveness to NaCl, so that the representation of glutamate was clearly different from that of NaCl. Further, the representation of glutamate was shown to be approximately as different from each of the other four tastants as they are from each other, as shown by multidimensional scaling and cluster analysis. Moreover, it was found that glutamate is approximately as well represented in terms of mean evoked neural activity and the number of cells with best responses to it as the other four stimuli, glucose, NaCl, HCl and quinine. It is concluded that in primate taste cortical areas, glutamate, which produces umami taste in humans, is approximately as well represented as are the tastes produced by: glucose (sweet), NaCl (salty), HCl (sour) and quinine HCl (sour).

Animals↗