Gyula Sáry

Human brain regions involved in visual categorization.

Categorization of dot patterns is a frequently used paradigm in the behavioral study of natural categorization. To determine the human brain regions involved in categorization, we used Positron Emission Tomography to compare regional Cerebral Blood Flow patterns in two tasks employing patterns that consisted of nine dots. In the categorization task, subjects categorized novel exemplars of two categories, generated by distorting two prototypes, and other random dot patterns. In the control task, subjects judged the position of similarly distorted patterns. Each task was presented at two matched levels of difficulty. Fixation of the fixation target served as baseline condition. The categorization task differentially activated the orbitofrontal cortex and two dorsolateral prefrontal regions. These three prefrontal regions were equally weakly active in the position discrimination task and the baseline condition. The intraparietal sulcus was activated in both tasks, albeit significantly less in the position discrimination than in the categorization task. A similar activation pattern was present in the neostriatum. Task difficulty had no effect. These functional imaging results show that the dot-pattern categorization task strongly engages prefrontal and parietal cortical areas. The activation of prefrontal cortex during visual categorization in humans agrees with the recent finding of category-related responses in macaque prefrontal neurons.

Functional Organization of Visual Cortex in the Owl Monkey

In this study, we compared the organization of orientation preference in visual areas V1, V2, and V3. Within these visual areas, we also quantified the relationship between orientation preference and cytochrome oxidase (CO) staining patterns. V1 maps of orientation preference contained both pinwheels and linear zones. The location of CO blobs did not relate in a systematic way to maps of orientation; although, as in other primates, there were approximately twice as many pinwheels as CO blobs. V2 contained bands of high and low orientation selectivity. The bands of high orientation selectivity were organized into pinwheels and linear zones, but iso-orientation domains were twice as large as those in V1. Quantitative comparisons between bands containing high or low orientation selectivity and CO dark and light bands suggested that at least four functional compartments exist in V2, CO dense bands with either high or low orientation selectivity, and CO light bands with either high or low selectivity. We also demonstrated that two functional compartments exist in V3, with zones of high orientation selectivity corresponding to CO dense areas and zones of low orientation selectivity corresponding to CO pale areas. Together with previous findings, these results suggest that the modular organization of V1 is similar across primates and indeed across most mammals. V2 organization in owl monkeys also appears similar to that of other simians but different from that of prosimians and other mammals. Finally, V3 of owl monkeys shows a compartmental organization for orientation selectivity that remains to be demonstrated in other primates.

Changes in the brain and core temperatures in relation to the various arousal states in rats in the light and dark periods of the day

In rats, brain temperature (Tbr) and core temperature (Tc) were recorded in parallel with the sleep-wake activity throughout the 24-h diurnal cycle, consisting of a 12-h light (L) and a 12-h dark (D) period. In order to characterize the temperature changes associated with the arousal states in the L and the D separately, (i) the average temperatures in wakefulness (W), non-rapid eye movement sleep (NREMS) and REM sleep (REMS), and at the transitions between the arousal states were calculated; (ii) the courses of temperatures before and after the transitions (falling asleep, awakening from NREMS or REMS, transition from NREMS to REMS) were determined; (iii) the rates of changes inTbr andTc were calculated for each state; and (iv) the correlations between the temperatures and the overall length of each arousal state, and betweenTbr andTc were studied.In both the L and D periods,Tbr andTc decreased at the beginning of NREMS, then levelled off, and increased slightly before awakening. Apart from short arousals which did not affect temperature,Tbr andTc increased in W, peaked 15–20 min after awakening, and declined significantly before the falling asleep. In REMS,Tbr increased at a high rate, while a slight increase inTc was evident in the L only. Correlations between the temperatures and the arousal states were found in both the L and the D. The courses ofTbr andTc were also correlated.The results support the existence of characteristic changes in body temperature related to the arousal states in the rat.

Effect of a serotonin agonist (sumatriptan) on the peptidergic innervation of the rat cerebral dura mater and on the expression of c-fos in the caudal trigeminal nucleus in an experimental migraine model

The supratentorial cerebral dura of the albino rat is equipped with a rich sensory innervation including nociceptive axons and their terminals, which display intense calcitonin gene‐related peptide (CGRP) immunoreactivity both in the connective tissue and around blood vessels. Stereotactic electrical stimulation of the trigeminal (Gasserian) ganglion, regarded as an experimental migraine model, induces marked increase and disintegration of club‐like perivascular CGRP‐immunopositive nerve endings in the dura. Intravenous administration of sumatriptan, prior to electrical stimulation, prevents disintegration of perivascular terminals and induces accumulation of CGRP in terminal and preterminal portions of peripheral sensory axons. Consequently, immunopositive terminals and varicosities increase in size; accumulation of axoplasmic organelles results in a “hollow” appearance of many varicosities. Since sumatriptan exerts its anti‐migraine effect by virtue of its agonist action on 5‐HT1D receptors, we suggest that sumatriptan prevents the release of CGRP from dural perivascular terminals by an action at 5‐HT1D receptors. In the caudal trigeminal nucleus electrical stimulation of the trigeminal ganglion induces, in interneurons, increased expression of the oncoprotein c‐fos which is not prevented by intravenous application of sumatriptan. Disparate findings regarding this effect are partly due to the fact that sumatriptan very poorly passes the blood‐brain barrier and partly to different experimental paradigms used by different authors. J. Neurosci. Res. 48:449–464, 1997.

On the impact of attention and motor planning on the lateral geniculate nucleus

Although the lateral geniculate nucleus (LGN) is one of the most thoroughly characterized thalamic nuclei, its functional role remains controversial. Traditionally, the LGN in primates has been viewed as the lowest level of a set of feedforward parallel visual pathways to cortex. These feedforward pathways are pictured as connected hierarchies of areas designed to construct the visual image gradually - adding more complex features as one marches through successive levels of the hierarchy. In terms of synapse number and circuitry, the anatomy suggests that the LGN can be viewed also as the ultimate terminus in a series of feedback pathways that originate at the highest cortical levels. Since the visual system is dynamic, a more accurate picture of image construction might be one in which information flows bidirectionally, through both the feedforward and feedback pathways constantly and simultaneously. Based upon evidence from anatomy, physiology, and imaging, we argue that the LGN is more than a simple gate for retinal information. Here, we review evidence that suggests that one function of the LGN is to enhance relevant visual signals through circuits related to both motor planning and attention. Specifically, we argue that major extraretinal inputs to the LGN may provide: (1) eye movement information to enhance and bind visual signals related to new saccade targets and (2) top-down and bottom-up information about target relevance to selectively enhance visual signals through spatial attention.

Depletion of calcitonin gene-related peptide from the caudal trigeminal nucleus of the rat after electrical stimulation of the Gasserian ganglion

Abstract Electrical stimulation of the Gasserian ganglion resulted in partial depletion of calcitonin gene-related peptide (CGRP) from ipsilateral central terminals of pseudounipolar primary sensory ganglion cells. Affected terminals exhibit decreased CGRP immunoreactivity as shown by cytophotometric densitomery of the caudal trigeminal nucleus. The decrease in CGRP immunoreactivity is statistically significant only in the medial one-third of the caudal trigeminal nucleus. Since earlier studies have shown that electrical stimulation of the Gasserian ganglion induces first accumulation then depletion of CGRP from perivascular sensory terminals in the dura mater, the present experiments suggest that CGRP is depleted also from central terminals of primary sensory trigeminal neurons, which might be of importance in the pathogenesis of migraine headache.

The representation of Kanizsa illusory contours in the monkey inferior temporal cortex.

Stimulus reduction is an effective way to study visual performance. Cues such as surface characteristics, colour and inner lines can be removed from stimuli, revealing how the change affects recognition and neural processing. An extreme reduction is the removal of the very stimulus, defining it with illusory lines. Perceived boundaries without physical differences between shape and background are called illusory (or subjective) contours. Illusory and real contours activate early stages of the macaque visual pathway in similar ways. However, data relating to the processing of illusory contours in higher visual areas are scarce. We recently reported how illusory contours based on abutting‐line gratings affect neurones in the monkey inferotemporal cortex, an area essential for object and shape vision. We now present data on how inferotemporal cortical neurones of monkeys react to another type of shapes, the Kanizsa figures. A set of line drawings, silhouettes, their illusory contour‐based counterparts, and control shapes have been presented to awake, fixating rhesus monkeys while single‐cell activity was recorded in the anterior part of the inferotemporal cortex. Most of the recorded neurones were responsive and selective to shapes presented as illusory contours. Shape selectivity was proved to be different for line drawings and illusory contours, and also for silhouettes and illusory contours. Neuronal response latencies for Kanizsa figures were significantly longer than those for line drawings and silhouettes. These results reveal differences in processing for Kanizsa figures and shapes having real contours in the monkey inferotemporal cortex.

A review on the inferior temporal cortex of the macaque.

In our review, we summarize recent advances in the research of the inferior temporal cortex (ITC) of the macaque monkey. This area of the cortex is known to have a crucial role in visual shape recognition and is regarded as being at the end stage of the so-called ventral visual pathway. In the last decade, several new findings appeared in the field without being integrated in a coherent view about the function, position, and operating principles of the area. During this decade, experimental techniques developed a great deal, and the way we look at the brain and brain function changed too. In this review, we try to integrate knowledge about the ITC to the changing view about the brain while outlining the work that has been done in the last decade.

Task-related modulation in the monkey inferotemporal cortex

The latencies of the neuronal responses from the inferotemporal cortical cells were analyzed in animals performing a visual fixation task and a recognition task with the same stimulus set. A consistent reduction in response latencies of about 10 ms was observed in favor of the recognition task. It was found that behavioral relevance reduces the latency in the inferotemporal cortex and it was concluded that behavioral significance accelerates information processing. This effect has not been described previously.

Static and dynamic views of visual cortical organization.

Without the aid of modern techniques Cajal speculated that cells in the visual cortex were connected in circuits. From Cajals time until fairly recently, the flow of information within the cells and circuits of visual cortex has been described as progressing from input to output, from sensation to action. In this chapter we argue that a paradigm shift in our concept of the visual cortical neuron is under way. The most important change in our view concerns the neurons functional role. Visual cortical neurons do not have static functional signatures but instead function dynamically depending on the ongoing activity of the networks to which they belong. These networks are not merely top-down or bottom-up unidirectional transmission lines, but rather represent machinery that uses recurrent information and is dynamic and highly adaptable. With the advancement of technology for analyzing the conversations of multiple neurons at many levels in the visual system and higher resolution imaging, we predict that the paradigm shift will progress to the point where neurons are no longer viewed as independent processing units but as members of subsets of networks where their role is mapped in space-time coordinates in relationship to the other neuronal members. This view moves us far from Cajals original views of the neuron. Nevertheless, we believe that understanding the basic morphology and wiring of networks will continue to contribute to our overall understanding of the visual cortex.