Prefrontal cortical unit activity and delayed alternation performance in monkeys

Laminar distributions of neurons sensitive to acetylcholine, noradrenaline and dopamine in the dorsolateral prefrontal cortex of the monkey

Sensitivities of neurons to acetylcholine (ACh), noradrenaline (NA) and dopamine (DA) were investigated at different depths of the dorsolateral prefrontal cortex (PFC) in awake or halothane-anesthetized macaque monkeys, using microiontophoretic techniques with multi-barreled electrodes. The laminar locations of tested neurons (n = 403) were estimated by reconstructing electrode tracks based on the microlesion made by passing a current through the recording barrel, which contained a carbon fiber. Iontophoretically applied drugs induced excitatory or inhibitory responses. Neurons excited by ACh (n = 105) were located mainly in layers III and V, and those inhibited by ACh (n = 126) were in layers III and IV. The majority of the NA-sensitive neurons (n = 123) were NA-inhibited neurons (n = 100), and were found most often in layers III and IV. The ratio of DA-sensitive neurons (excited, n = 74; inhibited, n = 63) to tested neurons was higher in the deep layers than in the superficial ones. These results indicate that sensitivities of the PFC neurons to ACh, NA and DA are not uniform between cortical layers, suggesting that each of these substances may predominantly influence the neuronal activity of particular layers of the monkey PFC.

Single neuron activity in dorsolateral prefrontal cortex of monkey during operant behavior sustained by food reward

The activity of 190 neurons was recorded from the dorsolateral prefrontal cortex of monkeys during an operant task that consisted of 3 phases: visual discrimination of food and non-food, bar pressing to gain access to the food and ingestion. In area 8, a fairly large proportion of the 49 recorded neurons responded in both the visual discrimination (37%) and motor initiation (35%) phases. Some functional heterogeneity seems evident within area 8 since visual discrimination responses were rostral, visuokinesis was central and motor initiation was in the caudal bank of the arcuate sulcus. Neurons in area 9 responded primarily (37%) during the bar pressing phase and less during the visual discrimination phase. Neurons in area 10 responded variously during most phases of the task--food discrimination, bar pressing, and ingestion. Neurons in the periprincipal sulcal area usually responded in the visual discrimination phase, but some which did not respond to food presented in front of the subject responded to meaningful visual or auditory cues that were related to food reward. The data suggest that neurons in the dorsolateral prefrontal cortex have multiple functions related to all phases of complex, learned feeding behavior. Functional roles of the prefrontal cortex and the lateral hypothalamus in development of feeding behavior are discussed.

Functional analysis of spatially discriminative neurons in prefrontal cortex of rhesus monkey

The present study addresses the question of whether prefrontal neurons that exhibit spatially selective patterns of discharge during the delay period in spatial delayed-response tasks code a mnemonic event. To examine this question, rhesus monkeys were trained to perform two variants of the classical spatial delayed-response task in both of which a delay intervened between cue presentation and response and the discriminative stimulus had to be recalled at the moment of response. They were also trained to perform two control tasks in which memory was not required since cues present throughout the delay informed the monkey of the correct response. Extracellular recordings were obtained from 192 neurons located in and around the principal sulcus of the frontal lobe during performance of both control and delay tasks. Comparison of the same neuron's activity across the 4 task conditions revealed a class of neuron that displayed spatially discriminative activity in the delay period only during delayed-response tasks and not during the same period of the control tasks. These neurons are candidates for units engaged in a central mnemonic process. Other neurons either exhibited similar activity in the delay period of control and delayed-response tasks or stronger discriminative behavior during this period in control tasks than in delayed response tasks. We conclude that delay-related spatially discriminative neurons found in the prefrontal association cortex are diversified and that certain of them play a specific role in mnemonic coding.

Responses of monkey prefrontal neurons during a visual tracking task reinforced by substantia innominata self-stimulation

The role of the substantia innominata (SI) in the generation of a behavior mediated by the prefrontal (PF) cortex was examined in two Japanese monkeys. PF neuronal activities related to a visual tracking task by wrist movement were recorded and neural responses to SI stimulation were analyzed. Sixty-six neurons showed task-related activity and were classified into 3 types. Type 1 (n = 31) showed transient activation during the movement. Type 2 (n = 26) showed gradually increasing activity before the reward presentation. Type 3 (n = 9) showed tonic activation from the GO signal to the reward presentation. Antidromic and orthodromic responses to SI stimulation were observed in every type. In Type 1, the percentage of antidromically activated neurons (26%) was similar to that of orthodromically activated ones (16%), but in Type 2, 83% of responding neurons showed antidromic responses, and 56% of Type 3 showed orthodromic responses. These results show that the different types of PF neurons have different anatomical relations to SI. Although orthodromically activated neurons were fewer than antidromically activated neurons, many orthodromically activated neurons showed movement-related activity. This suggests that ICSS at SI facilitates the generation of the behavior through the afferent pathway to PF.

Delay-related activity of prefrontal neurons in rhesus monkeys performing delayed response

Activity of dorsolateral prefrontal cortical neurons was examined in rhesus monkeys while they performed a spatial delayed-response task with delays of 2, 4, 8 or 12 s interposed between cue and response. Of the 600 neurons recorded for at least 10 trials under each delay condition, 95 displayed a pattern of discharge during the delay period which was significantly different from neuronal firing before or after this period. Changes in the duration of the delay elicit two distinct patterns of activity in these neurons: some (59/95, 62%) exhibit a fixed pattern of discharge regardless of the duration of the ensuing delay; others (31/95, 33%) alter their pattern of activity in relation to the temporal changes. Although both types of delay-related neurons display a variety of discharge profiles, more than half of each class exhibit their highest activity in the early part of the delay period. A related finding concerns a small subclass of spatially selective neurons which fire significantly more when the cue is presented on the left than on the right or vice versa. A striking 80% of these spatially discriminative neurons exhibit peak activity in the first few seconds of the delay period. These findings provide cellular evidence that (1) prefrontal neurons are responsive to temporal as well as spatial features of the delayed-response task; and (2) the involvement of a subset of these is particularly critical in the first few seconds of the delay. The latter finding emphasizes that prefrontal neurons may play an important role in the registration process of spatial memory.

Prefrontal unit activity of macaque monkeys during auditory and visual reaction time tasks

During a simple reaction time task using auditory or visual stimuli, a total of 96 single units were recorded from the dorsolateral prefrontal cortex of macaque monkeys. These monkeys were trained to depress a lever for a fixed period which produced a tone burst or a small spot of light. After a variable period, the stimulus intensity changed, and then, the monkey released the lever. Eighty-one cue-related units were classified into 3 types according to their decay time; that is, phasic, tonic and mixed. Phasic units (n = 19) showed a transient increase of discharge rate with a relatively short peak latency (70-300 ms). Of these, 17 units responded exclusively to either visual or auditory stimuli and two to both. Tonic units (n = 55) showed enhanced or suppressed activity, with longer latencies, which was sustained as long as the cue period continued. The temporal pattern of the discharge in 23 tonic units was found to be similar for both the auditory and visual cues. Seven mixed-type units showed combined phasic and tonic patterns. Lever release-related units (n = 15) were activated only during the period of lever release with no distinction in cue modality. It is suggested that the dorsolateral prefrontal cortex receives sensory inputs fairly discretely on the phasic-type neurons and that these sensory activities are transmitted to the tonic-type neurons which lead to an initiation of the lever release behavior.

Thalamic mediodorsal nucleus and memory: a critical evaluation of studies in animals and man

Following a general description of the anatomical organization of the thalamic mediodorsal nucleus (MD) of animals and man, the involvement of this nucleus in the processing of memory related information has been evaluated by reviewing stimulation, electrophysiological, and lesion studies in animals, and by reviewing research on induced lesions, degenerative changes and vascular damage of MD in humans. Neither the results from animal experiments nor those from studies on humans provide clear-cut evidence for a specific, memory related role of MD. However, the findings here presented do support the theory that MD is one of several, possible memory related relay stations. While therapeutically induced and circumscribed lesions of MD rarely result in long-lasting memory deficits, pathological processes in MD are more likely to be followed by severe memory disturbances if one or more particular structures in addition to MD are included in the lesioned regions. Consequently, it is emphasized that only the disruption of more than one site along memory related pathways will result in severe and enduring memory deficits. To account for apparent inter-species differences in the involvement of MD in memory related processes, it has been argued that MD and its principal cortical target region might basically be involved in arousal and emotional processes, but that for primates and especially for man the phylogenetically young parvocellular sector of MD and its cortical projection region, the dorsolateral prefrontal cortex, are furthermore involved in memory functions, which are modulated by emotional factors via the rest of MD and the prefrontal cortex.

Prefrontal unit activity during a color discrimination task with GO and NO-GO responses in the monkey

The activity of single neurons was recorded in the prefrontal cortex of monkeys performing a color discrimination GO and NO-GO task with a sequenced 3 visual stimuli (starting signal of a trial, discriminanda and command for response selection, respectively). A total of 161 units showed changes in their activity in association with at least one of the 4 events of the task (3 visual events and reward). Three patterns of activity change coupled to event onset (Type A, B nd C) were found irrespective of the kind of events. Type A was a transient activity increase after event onset. Fifty-two units showed the Type A change. Three-fourths of them became active after one particular event, and the remaining one-fourth after more than one event. Type B was a gradual activity increase preceding event onset followed by a decrease after event onset. Sixty-five units showed the Type B change. In two-thirds of them, the activity changes occurred around one particular event, and in the remaining one-third around more than one event. Type C consisted of sustained activity between two different events. Eighty-three units showed the Type C change. This type of change was found in 5 different combinations of events. Four-fifths of the 161 units showed one of 3 patterns, and the remaining one-fifth more than one of these patterns during different periods of a trial. It is suggested that Types A and B are involved in the information processing in relation to a particular event onset, and Type C provides a reference for distinction of events.

Prefrontal unit activity during delayed conditional discriminations in the monkey

Single unit activity was recorded from the prefrontal cortex (principalis, arcuate and inferior convexity areas) of the monkey while the animal was performing a delayed conditional discrimination task. Sequential events of the task were as follows: instructional cue presentation, 1st delay, presentation of two pattern stimuli on left and right windows respectively as a discriminative cue, 2nd delay and choice response to the left or right window. The positive pattern was dependent on the instructional cue. In total, 424 units obtained from two monkeys showed a correlation with some aspects of the task. Of these 424 task-related units, 169 differential between the instructional cues and/or the discriminative cues. The majority of these differential units (n = 123) were found to be related to spatial information processing (related to the side of the response) while 19 differential units were related to non-spatial information processing (related to the color or pattern configuration of the cue). The activity of the remaining 27 differential units was considered to be related to both spatial and non-spatial information processing (related to both the instructional cue and the side of the response) and this type of unit was shown to be involved in conditional information processing. The results indicate that prefrontal units may be related to the meaning of the stimulus independent of its physical properties.

Radioactive 2-deoxy-D-glucose incorporation into the prefrontal and premotor cortex of the monkey performing a forelimb movement

Radioactive 2-deoxy-D-glucose (2-DG) incorporation into the monkey prefrontal and premotor cortex was studied in relation to extention-flexion movement at the wrist joint in two experimental and two control monkeys. With 2-DG injection and 45 min' intensive task performance thereafter, the following areas showed increased accumulations of radioactive glucose: the dorsomedial and dorsolateral prefrontal areas, including the lateral and medial banks of the principle sulcus; the ventral prefrontal and orbitofrontal areas; the cingulate gyrus, and the premotor cortex. In these areas, patch- or strip-like patterns were observed in the accumulation of 2-DG.

Neuronal activity in the monkey dorsolateral prefrontal cortex during a discrimination task with delay

Ninety-nine single neuron activities of the dorsolateral prefrontal cortex of 3 monkeys were recorded during performance of a Konorski task. Green or red lights were presented successively with a separation of fixed delay interval. The monkey responded as soon as the second stimulus was presented. If the two stimuli were color-matched, the 'YES' lever press was rewarded; if the two stimuli were not, the 'NO' lever press was rewarded. In the second task, after paired color stimuli, a tone pip was presented as the 'GO' signal for lever presses. During sample and matching periods 50 neurons increased their discharge rates and 10 decreased. In 86% of increasing type neurons rate increase occurred during both periods. During auditory GO periods, 27 neurons increased their rates and 11 decreased. Discharge peak was before or at the moment of hold key release. In 60% of these neurons were also observed the rate changes to sample and matching stimuli. Differential activations between left and right levers were found in 20%. It was suggested that the prefrontal cortex is related to a sensorial attention mechanism to the visual stimulus which enables correct choice of the behavior to be rewarded.

Prefrontal and cingulate unit activity during timing behavior in the monkey

Single unit activity was recorded from the dorsolateral prefrontal cortex and the anterior cingulate cortex while monkeys were performing a modified differential reinforcement of long latencies (DRLL) task. A total of 252 prefrontal units and 218 anterior cingulate units showed an obvious change in discharge rate (increase or decrease) in association with one or more of the events of a DRLL task. Related units were classified into 3 main groups: S--R event units, reward-error units, and timing units. S--R event units consisted of three subtypes: stimulus-related, response-related, and stimulus--response-related units. Reward-error units contained reward-related units and error-recognition units. Error-recognition units showed a vigorous increase in firing only after incorrect responses. These units were also responsive to omission of reinforcement on correct trials. Three types of timing units were distinguishable. The first one showed an anticipatory change prior to stimulus onset, and the second one exhibited a gradual anticipatory change preceding the time of responding. The third one manifested a sustained change during delay and an abrupt cessation of change in firing at the time of response initiation.

Afferent and efferent subcortical projections of behaviorally defined sectors of prefrontal granular cortex

Although the functional significance of the midprincipalis region is well known, the afferent and efferent connections of this zone, in comparison to the anterior and posterior portions of the cortex lining the principal sulcus, are poorly understood. In 3 animals the retrograde tracer HRP and the anterograde tracers, tritiated proline, lysine and leucine, were injected into the sulcal cortex lining the principal sulcus. The cortex forming the banks of the principal sulcus was divided into anterior, middle and posterior sectors with one animal used for each zone. As expected from previous studies, the heaviest afferents to the cortex forming the principal sulcus were from the parvocellular portions of the medical dorsal nucleus. The medial pulvinar nucleus and the nucleus limitans projected to only the anterior and posterior portions of the cortex lining the principal sulcus. Projections were seen to all 3 sectors from the anterior, midline, intralaminar and lateral thalamic nuclei. Although cells were seen in the hypothalamus following injections in all 3 sectors of the cortex lining the principal sulcus, the heaviest hypothalamic projections were noted after injections into the mid-sector of the cortex. These HRP-positive cells were in the dorsal and lateral hypothalamic area, dorsal medial nucleus and in the lateral mammillary nucleus. These findings link the midprincipalis region with the prefrontolimbic circuit, and suggest that the midprincipalis region, n. medialis dorsalis, the mammillary bodies and perhaps the cingulate gyrus constitute part of an anatomical circuit concerned with memory processes.

Single unit activity in cat prefrontal and posterior association cortex during performance of spatial reversal tasks

Relationships between the performance of complex learning tasks and the firing patterns of single unites in prefrontal and posterior association areas of the neocortex were studied in 9 cats. Recording data from 2 additional cats served as controls for sensory and motor aspects of the tasks employed. During recordings, animals sat in a box, their heads fixed to a stereotaxic instrument, and performed a non-sensory spatial reversal or delayed-alternation task by pressing 1 of 2 lateralized retractable levers. Units obtained were classified into 5 basic types according to their correlation with aspects of the tasks. The first 3 types were related directly to sensory, motor and motivational aspects of the tasks. The fourth type was named associative, as it contained units which changed firing rates in advance or as a consequence of environmental events, rather than in direct relation. Discharge changes of units of the fifth type were rated as non-specific or non-specifiable. Between recording areas and tasks a striking similarity of unit firing patterns was obtained. Differences were observed only between the reactivity of polymodal cells in prefrontal and in posterior fields, and in the proportions of units revealing mnemonic and motor response aspects. It was concluded that a considerable functional overlap between prefrontal and posterior association areas of the cat with respect to processing of complex learning tasks exists on the single neuron level.

Integration of cerebral and peripheral inputs by interpositus neurons in monkey

The patterns of convergence of cerebral and peripheral nerve inputs onto interpositus neurons were studied in cebus monkeys. The strongest inputs to interpositus neurons are from motor and somatosensory cortex, with weaker inputs from peripheral nerves and cerebral area 6. The neurons in the anterior portion of interpositus receive cerebral and peripheral inputs primarily representing the hindlimb, while inputs to neurons in the posterior division represent forelimb or mixed forelimb and hindlimb. The hindlimb neurons integrate signals principally from motor cortex, somatosensory cortex, nerves, supplementary motor and medial pre-motor areas, while forelimb neurons receive inputs from motor, somatosensory, lateral premotor cortical areas and nerves. The results from this study are compared with those from studies of interpositus and dentate neurons in cat and monkey in order to determine the role of n. interpositus in movement. It is suggested that the inputs integrated by interpositus neurons are consistent with a role in up-dating skilled movements.

Prefrontal unit activity and delayed response: relation to cue location versus direction of response

Two monkeys were trained to perform 3 kinds of spatial tasks (right-left DR, up-down DR, and conditional position discrimination with delay). In the conditional position task, the animal was required to respond to the right (left) choice key when the cue had been presented on the upper (lower) position. Single unit activity was recorded from the principalis area of the dorsolateral prefrontal cortex while the animal was performing the tasks. Once a differential delay unit (i.e., unit showing specificity during the delay period for two kinds of trials) was found in right-left DR (or up-down DR), the same unit was tested in the other tasks. A total of 32 differential delay units were investigated sufficiently to allow comparison of all 3 conditions. The results indicate that there are two types of differential delay units. The first type showed a clear dependence on the cue location, while the second type was related to the direction of the impending response. Activity of the second type of differential delay unit during the delay period served to predict the occurrence of errors, whereas this relation between unit discharge and correct or incorrect responses was not seen for the first type of differential delay unit.

Single unit activity in basal ganglia of monkeys during performance of a delayed response task

Single unit activity was recorded from the basal ganglia (caudate, putamen and globus pallidus) of monkeys during the performance of a delayed-response task. The task was divided into five epochs: stimulus onset, delay, pre-response, post-response and reqard. A high percentage of units recorded from the basal ganglia were found to show significant changes in activity during one or more epochs. Examination of the proportion of units excited or inhibited during a particular epoch indicated that brief increases or decreases in unit firing rates occurred "in phase" in both pallidum and caudate. Longer lasting firing rate changes, however, tended to occur in opposite directions in these two structures. This latter finding is interpreted as representing the consequence of persistent increases or decreases in activity of inhibitory interneurons in the caudate nucleus.

Afferents to prefrontal cortex from the thalamic mediodorsal nucleus in the rhesus monkey

Thalamic afferents to Macaque prefrontal cortex from the mediodorsal nucleus were examined by techniques specific for anterograde degeneration and axoplasmic transport. The sampling procedure employed permits establishing the extent of topographic projections to cortex from subcortical foci for the same brain which was surveyed subsequently in tracing specific neuronal connections by electron microscopy. Topographic and general laminar distribution of thalamic terminals are presented in terms of 3 subareas of prefrontal cortex. The dorsolateral and ventral (orbital) surfaces of prefrontal cortex receive respectively projections from the lateral and medial subdivision of the mediodorsal nucleus. In addition, the medial wall of the frontal lobe, including the dorsomedial part of the lateral convexity, heretofore regarded as athalamic, receives input from the caudal-dorsomedial aspect of the mediodorsal nucleus. Preliminary evidence suggests that axons from the mediodorsal nucleus terminate in the head of caudate nucleus, as Sachs-81 described 65 years ago in the first orthograde study of thalamo-prefrontal cortex connections.