The cryoloop: an adaptable reversible cooling deactivation method for behavioral or electrophysiological assessment of neural function.

We describe a very adaptable reversible inactivation technique for the behavioral or electrophysiological analysis of neural circuits. The cryoloop device can be permanently implanted or topically applied in an acute preparation to apply cold to discrete surface regions of the central nervous system (e.g. cerebral cortex or midbrain). The cryoloop consists of a custom shaped, stainless steel, hypodermic tubing and cooling is effected by passing chilled methanol through the lumen of the tubing. Cryoloop temperature is monitored by a microthermocouple attached to the union of the loop, and can be maintained within +/- 1 degrees C of a desired temperature. In chronic preparations, implanted cryoloops have been maintained in cats and monkeys for periods in excess of 2 years. After this period there are no structural, metabolic of functional changes in the deactivated tissue, and full reversibility of cooling-induced effects is maintained. Operation of multiple cryoprobes provides great flexibility of experimental protocols, permits double and triple functional dissociations to be made, and strengthens experimental design considerably.

Perception, learning and identification studied with reversible suppression of cortical visual areas in monkeys.

We use cold to reversibly suppress cortical areas involved in visual perception, learning and retrieval and we found a localization of functions essential for performance of delayed match-to-sample (DMS) in anterior ventral temporal cortex, we call ventral TE (TEv). We also found a visual input for this area that is separate from the one going to the heart of inferotemporal cortex and suppressing this input also impairs performance of DMS. Suppressing the dorsal half of TE (TEd) disrupts retrieval of some, but not all complex images, and different images are disrupted in different animals. This variability within and between animals is extreme, with perfect performance on some complex images and below chance on others. We suggested that TEd represents some, but not all elements of the images. In attempting to discover what those elements might be, we found that TEd suppression disrupts the perception of small figures, but not the larger figures that they compose. We also found that it impaired the discrimination and matching of colors, without impairing the ability to detect and differentiate hues. We proposed that TEd represents the details and colors of things, but not global figures. Also, complex objects do not have a representation in one area, rather its representation involves the entire visual system, including TE with different elements of the image represented in different parts.

Local and global perception examined by reversible suppression of temporal cortex with cold.

We placed cryodes over both sides of the dorsal inferotemporal cortex (TEd) in three monkeys in order to suppress its functions during perception of compound visual images. We used three pairs of images in a concurrent visual discrimination: 1. Congruent pair, a large T made of small Ts and a large 7 made of small 7s with response to the large 7 rewarded. 2. Incongruent pair, a T made of small 7s and a 7 made of small Ts with response to the large 7 rewarded. 3. Random pair, a scattering of small Ts in one stimulus and small 7s in the other with response to the small 7 rewarded. If TEd processes the global but not the local elements of a visual figure, animals with TEd suppressed should see only the local figures and they would fail only on the incongruent stimulus. Alternatively, if TEd processed the local, but not the global elements, they should fail only on the scattered small figures where there was no global element as a cue. Finally, if TEd suppression impairs discrimination of forms, they should be impaired on all the figures because they were the same form, but different size. We found that during TEd suppression, the animals failed on the scattered small elements, but not on the global figures formed of the small elements.

Retrieval of color and form during suppression of temporal cortex with cold.

Five cryodes were implanted on each side over the dorsal aspect of inferotemporal cortex (TEd) of three monkeys. They were trained on a form discrimination and three color discriminations. Suppression of TEd with cold disrupted retrieval of the color, but not the form discriminations. The animals could find the colors in a background of shifting values of gray, indicating that the suppression did not reduce their color perception to gray. They initially had great difficulty matching red to red and green to green, although that recovered with experience. The animals tended to respond to one or the other of the colors, indicating that they could perceive and discriminate them, but, either lost information about the correct stimulus, or something from past experience was interfering with performance. We suggested that cooling TEd suppresses new and recent learning of color discriminations, but it does not suppress some previous experience that intrudes upon performance of new tasks. TEd might contain episodic information about colors necessary for performance of the immediate task.

Some comments on the special cognitive functions claimed for the hippocampus.

This paper challenges the idea that memory is the special function of any single brain structure, an idea that developed from clinical cases of amnesia that had lesions in and around the hippocampus. There are many instances of amnesia and other evidence of memory functions in brain areas that do not involve the hippocampus. The evidence that medial temporal lobe lesions in animals produce uncontaminated memory deficits is reviewed and rejected, as well as the evidence that the memory task used in these experiments has any special relationship to the hippocampus. It is proposed that memory is not the function of any one structure, but is a part of local neuronal operations carried out in all cortical areas where the information to be remembered is processed and perceived. If this suggestion is correct, then a local lesion will cause a loss of local function and the memory for that function.

The effects of number of stimuli and prior exposure on performance of concurrent visual discriminations during suppression of inferotemporal cortex with cold.

The visual part of the temporal cortex, cytoarchitectural area TE has been split into dorsal (TEd) and ventral (TEv) subdivisions. TE has long been associated with the identification of objects. However, in order to explain retrieval deficits with suppression of prestriate cortex, but not with suppression of TE, we hypothesized that object identification might take place in a working memory in the prestriate cortex upstream from TE. Exposure to the stimuli before suppressing TEd was hypothesized to cause its contribution to be relayed to prestriate cortex in anticipation of further work with them. This predicts that during TEd suppression there is a loss of access to some long-term visual memory, and without that access, large numbers of stimuli should overwhelm the limited capacity working memory. It also predicts that prior exposure to stimuli should protect them from loss during TEd suppression. We challenged these predictions in two experiments. In the first, we tested the animals on three concurrent discriminations requiring them to retrieve 8 pairs of stimuli for each one. The animals performed well on some of the discriminations during TEd suppression, but failed on others, which is consistent with the prediction. Also, different animals failed with different stimuli. However, when we tested with only the failed discriminations, they still did badly with one or two pairs, which is not consistent with the prediction. In the second experiment, we tested them with 1, 4, 7 or 10 pairs of visual discriminations drawn from a set of 23 the animals had learned. Half of the discriminations were presented immediately before suppression and half were not.(ABSTRACT TRUNCATED AT 250 WORDS)

Retrieval of a face discrimination during suppression of monkey temporal cortex with cold.

Cells in the monkey temporal cortex that respond selectively to faces suggest that monkeys might have a brain structure similar to that in humans where lesions produce prosopagnosia, but effects of lesions on retrieval of face discriminations have been ambiguous in monkeys. It is possible that the stimuli in the monkey experiments were contaminated with non-face elements that could be discriminated by other parts of the visual system. In this experiment we modified the image of a monkey face creating two faces that were identical except for their internal features. We trained monkeys to discriminate these faces and then reversibly suppressed the inferotemporal cortex with cold and tested their ability to recall them. Cooling the temporal cortex produced a severe impairment in retrieval of the discrimination that remained constant across six 40-trial replications.

Retrieval of active and inactive visual discriminations while temporal cortex is suppressed with cold.

While local cooling of inferotemporal cortex (IT) impairs new visual learning, it has little effect on recall. However, in visual discriminations, there is typically extensive exposure to the stimuli before cortical inactivation. Perhaps if recall was prevented before suppression, it would fail during suppression. Three animals with cryodes covering a major part of IT were trained on two face discriminations. They were then run on one of these discriminations for 3 days to create the expectation that the task would be continued with the same stimuli, and on the 4th day, they were started with these stimuli, but after cold suppression, they were switched to a discrimination that they should not have anticipated. IT suppression prevented recall of the discrimination that had not been pre-exposed; performance dropped to chance and stayed there for 50 trials. When they were switched back to the initial pair, performance returned nearly to normal. The experiment was repeated with the role of anticipated and unanticipated stimuli reversed. It was suggested that pre-exposure to the discrimination created the expectation that the same stimuli would continue to be used, and induced information about them to be copied from IT into prestriate cortex.

Cortical afferents to behaviorally defined regions of the inferior temporal and parahippocampal gyri as demonstrated by WGA-HRP.

The inferior temporal gyrus in the monkey appears to be unique among the many extrastriate visual cortices in its importance for normal performance of delayed match-to-sample, a visual memory task. However, the anatomical pathway providing visual information to this portion of the temporal lobe remains unclear. In this study, wheat germ agglutinin conjugated to horseradish peroxidase (WGA-HRP) was injected into the anterior inferior temporal gyrus and heavy projections were found to arise in cytoarchitectural area TF of the parahippocampal gyrus, as well as moderate projections in more posterior portions of inferior temporal gyrus and perirhinal and entorhinal cortices. Subsequently, WGA-HRP was injected into area TF, resulting in retrogradely labeled cells primarily located in the portions of area TF adjacent to the injection and also in the occipitotemporal sulcus including the ventral portion of the prestriate visual area V4. Moderate projections were found to originate from the dorsal region of area V4 in the lunate sulcus, portions of the caudal parietal lobe, the posterior bank of caudal superior temporal sulcus, and area OPT located at the tip of the superior temporal sulcus. The middle temporal gyrus, foveal prestriate cortex, and area TEO, a transitional area between temporal and occipital visual areas, were all free from retrogradely labeled cells. These latter areas are included in the well-established anatomical system that is known to carry visual information from striate cortex through prestriate to eventually reach dorsal portions of inferotemporal cortex which is coincident with the temporal lobe visual area TE. It is suggested here that there is an additional ventral pathway into area TE as well, which includes projections through portions of the prestriate cortex, occipitotemporal sulcus, and parahippocampal gyrus, ultimately reaching the anterior inferior temporal gyrus, an area that may be specialized to hold visual information over brief periods of time.

Reversible cold lesions of the parahippocampal gyrus in monkeys result in deficits on the delayed match-to-sample and other visual tasks.

Two bilateral cooling probes were placed over the parahippocampal gyrus (pg) and the cortex just dorsolateral to it, the posterior inferotemporal gyrus (p.itg) in 4 Macaca fascicularis. Behavioral tests included: delayed match-to-sample (DMS); the acquisition and retention of single visual discriminations; the acquisition and retention of a concurrent visual discrimination task; and the retention of a spatial reversal task. During cooling of the pg and of the pg and p.itg together, there was a deficit at all delays on DMS. For both the single and concurrent visual discriminations, pg cooling produced an acquisition but not a retention deficit, although the acquisition deficit for the concurrent task was not significant at the 0.05 level. Cooling of p.itg had no significant effect on these tasks. No cooling had any affect on the spatial reversal task. It was concluded that pg serves as an important visual input into the anterior half of the itg for performance of DMS and the acquisition of visual discriminations. For several reasons, it was argued that the deficits were not caused by cooling of the hippocampus.

An experimental test of the theory that visual information is stored in the inferotemporal cortex.

This experiment employed reversible cold lesions to assess the possible storage functions of inferotemporal cortex (IT) for visual information. Four Macaca fascicularis were chronically implanted with 4 bilateral sets of cryodes which covered dorsal and ventral IT. Animals learned visual discrimination problems while subsections of IT were cooled. Retention was then tested with the previously warm tissue cold as well as with all of IT cold. In addition, an attempt was made to replicate previous studies showing spared retention of visual discriminations with preoperative overtraining. When animals learned a visual discrimination with partial bilateral IT cooling, retention was good when the previously warm tissue was cooled. If learning occurred with partial IT cooling confined to a single hemisphere, retention was lost when the previously warm tissue was cooled. When acquisition occurred without any cooling, retention was severely impaired when IT was cooled, even if animals received 1000 trials of overtraining. The results are attributed to distributed stimulus-analyzing properties of IT.

Lesions of the anterior temporal stem and the performance of delayed match-to-sample and visual discriminations in monkeys.

Resection of the medial temporal lobes in humans produces an anterograde amnesia in which past memories are seemingly intact, but the ability to form new memories is compromised. Efforts to reproduce these symptoms in animals have relied extensively on the delayed non-match-to-sample (DNMS) and the delayed match-to-sample (DMS) tasks. DNMS deficits have been found with combined damage to the amygdala and hippocampus, but not to the adjacent white matter (the temporal stem) that connects the temporal cortex to other brain areas. DMS deficits are, however, produced by lesions to either the anteroventral temporal cortex or the orbital frontal cortex. These two areas are interconnected through the anterior temporal stem. The present study examined the hypothesis that an anterior temporal stem lesion would impair DMS in monkeys. The anterior extreme of the temporal stem was transected in 4 Macaca fascicularis and resulted in a powerful deficit on DMS at all delays. Postoperative retention of preoperatively learned visual discriminations and postoperative learning of new visual discriminations were not reliably impaired.

Retention deficits produced in monkeys with reversible cold lesions in the prestriate cortex.

Cryodes were implanted over the prelunate and fusiform gyri in 4 monkeys. The intention was to block the prestriate projection to the inferotemporal cortex (IT). Cooling this cortex produced deficits that were completely reversed by removing the cold. The deficits appeared in the recall of visual discriminations that were learned prior to the application of the cold, but new discriminations were learned during cooling at the same rate as in control animals. This closely resembles the results of ablation experiments in this same cortex. There was no deficit when cooling this area on a delayed match-to-sample (DMS) task that required the animals to hold visual information over delays of 10 or 45 s. However, performance was close to chance during cooling when the delays were 6 min. With this long delay, when cooling was done separately at sample when the stimulus is received, or at match when the information is recalled, a significant deficit occurred only at match. The results are consistent with the suggestion that this area is involved in retrieval of visual information from long-term memory.

The performance of visual tasks while segments of the inferotemporal cortex are suppressed by cold.

Cold was used to suppress the function of subdivisions of the inferotemporal cortex. Three cryodes were placed bilaterally, one over the lower bank of the superior temporal sulcus (sts), one over the middle temporal gyrus (mtg) and one over the inferior temporal gyrus (itg). The animals were tested with delayed match-to-sample (DMS) and simultaneous visual discriminations. The DMS required the animal to recall a projected image of an object over delays of 0, 15, 30 and 45 s. The 3 cryodes were cooled separately during the performance of the DMS and only itg cooling produced a deficit. This was compared to the effects of ablative bilateral lesions; damage to itg but not mtg disrupted performance of DMS. The greatest deficit was in an animal with a small lesion in the ventral pole and anterior extreme of itg. Cooling individual cryodes was without effect on a discrimination between horizontal and vertical stripes, but produced a significant deficit from each of the 3 placements on a discrimination between monkey faces. Chance performance on all visual discriminations resulted from cooling all cryodes. Unilateral cooling of all cryodes produced significant effects on the face discrimination, but there was no significant difference between the two sides in the severity of the deficit.

Visual learning suppressed by cooling the temporal pole.

Three monkeys were trained to remember colored photographs of objects over delays of 0, 15, 30, and 45 s. Then two pairs of cooling devices were implanted bilaterally over the anterior 9 mm of the temporal lobe. The devices consisted of 3 X 10 mm loops of stainless steel tubing into which cooled methanol could be pumped. One pair (anterior pair) covered the medial part of the temporal tip (area TG), starting at the rhinal sulcus and extending 3 mm laterally. The second pair (posterior pair) was placed 3 mm lateral to the anterior pair, covering the rest of TG and the anterior extreme of the inferotemporal gyri, anterior TE. Cooling either pair of probes produced a deficit at all delays, but the deficit was greater at the longest delays. There was no difference between cooling the anterior pair and cooling the posterior pair except that cooling the anterior pair greatly increased the disruption of recall that is produced by an interfering stimulus. When all four probes were cooled, which suppressed the function of the entire temporal tip, performance dropped to chance at all delays. While under this condition, the animals could not learn new visual discriminations but could perform previously learned visual discriminations. These results are consistent with the suggestion that the temporal pole is the store for the brief anterograde memory that is available to the medial temporal amnesics.

Neocortical projections of the rat anterior commissure.

The posterior limb of the rat anterior commissure (ACp) was studied in order to better define a temporal cortex in this species. Two methods were used: (1) the anterior commissure (AC) was destroyed on one side of the midline, and the resulting anterograde degeneration was traced with the Fink-Heimer stain; and (2) the corpus callosum and hippocampal commissure were severed, but the AC was left intact. [3H]-Leucine and proline were injected into one hemisphere, and the transported label was traced into the contralateral hemisphere with autoradiography. ACp was found to project to the contralateral cortex along the rhinal sulcus. The pyriform cortex received a projection along the entire length of the sulcus. There was also a distinct projection to neocortex on the lateral surface above the rhinal sulcus which appears to be analogous to the temporal cortex projection of ACp in other species. This finding of a neocortical projection of the ACp in the rat is consistent with observations that have been made on other mammals.

The neuroanatomy of amnesia. A critique of the hippocampal memory hypothesis.

The discovery that medial temporal lobe lesions produce amnesia in humans if the lesion extends sufficiently far posteriorly to include the hippocampus forms the keystone of the hippocampal memory hypothesis. Strong supporting evidence comes from the occurrence of mammillary body disease in Korsakoff's psychosis. Disease of the posterior cerebral artery confirmed the observations on the medial temporal lobectomies by showing that pathology in the ventromedial quadrant of the temporal lobe produces amnesia. The occasional piece of contradictory evidence was sufficiently ambiguous to be dismissed or re-interpreted. Although the contradictory evidence that emerged from animal research created severe difficulties, opinion had crystallized on the matter to the degree that the data were unable to force rejection of the hippocampal memory hypothesis. This necessarily led to the conclusion that the animal model is a poor one: either the human hippocampus is unique with respect to memory or the tests which are used in animals do not tap the same mnemonic processes that are impaired by the human lesions. Both these arguments are nearly impossible to refute. The brain of every species is different and there is no way in which monkeys and humans can be tested under identical conditions. There has never been much enthusiasm for the suggestion that the human hippocampus is so different from other animals that this uniqueness could account for the apparent differences between the behavioural effects of human and animal hippocampal lesions. However, many experimenters have devised clever tests of the possibility that the problem is in the animal behavioural measures. Given sufficient circularity of reasoning, the project must necessarily eventually be successful. The argument is that if the usual tests of learning and memory that are used with animals are not disrupted by hippocampal lesions, then these are not tests of the kinds of learning and memory defects displayed by human amnestics. One has only to search for tasks that are disrupted by hippocampal lesions in animals, and these then must tap the same memory processes that are disrupted by the human lesions. The possibility has rarely been seriously considered that it might be damage to some structure in the ventromedial quadrant of the temporal lobe other than the hippocampus that is responsible for the amnesia. The amygdala and entorhinal area have been ruled out by both the human and animal data. However, the temporal stem is a likely possibility. Its position makes it vulnerable to the surgical approach which was used in human medial temporal lobectomies, and its damage in animals produces deficits in learning and retention. When medial temporal lesions were made in monkeys in the same way that they were made in humans, inadvertent damage to the temporal stem occurred along with the intended amygdaloid and hippocampal injury. Symptoms characteristic of damage to the temporal cortex resulted from these lesions and they were probably caused by the damage to the stem...

Visual discrimination impaired by cutting temporal lobe connections.

An attempt was made to transect the white matter that connects the anterior temporal lobe with dorsal and medial brain areas. Eight monkeys were trained preoperatively on a visual discrimination and tested postoperatively for retention and relearning of the task. They were also tested for Klüver-Bucy symptoms. The two animals that had complete lesions were unable to relearn the visual discrimination. It is suggested that human medial temporal lesions may produce their effects on learning and retention by damage to temporal white matter rather than by destruction of hippocampus.

Cortical blindness after overlapping retinal-striate lesions: a limit to plasticity in the central visual system.

Lesions in cats, rats, and monkeys that spare more than 2% of the optic tract or visual cortex cause trivial deficits on most measures of vision. Overlap in the topography of the visual system may allow the spared remnant to 'see' a wider field of vision than the physiological map predicts. We tested whether monkeys left with only the lower retinal-field portion of their striate map could see with information coming from the upper half of the retina. In 6 rhesus monkeys the ganglion fibers exiting from the lower half of both retinae were cut with a photocoagulator. Later, the portion of area 17 which, according to the electrophysiological map, controls upper retinal vision, was ablated bilaterally. The combined retinal and striate lesions overlapped to include the entire visual field. Together they produced cortical blindness. The monkeys' performance of two pattern and object tasks remained at chance throughout the survival period. A previous study has described considerable sparing of vision after combined optic tract and visual cortex lesions in cats. Differences in the lesion methods and in the anatomy of the cat and monkey visual system may explain the disagreement.