A metabolic mapping study of orientation discrimination and detection tasks in the cat

Visual pathways serving motion detection in the mammalian brain

Motion perception is the process through which one gathers information on the dynamic visual world, in terms of the speed and movement direction of its elements. Motion sensation takes place from the retinal light sensitive elements, through the visual thalamus, the primary and higher visual cortices. In the present review we aim to focus on the extrageniculo-extrastriate cortical and subcortical visual structures of the feline and macaque brain and discuss their functional role in visual motion perception. Special attention is paid to the ascending tectofugal system that may serve for detection of the visual environment during self-motion.

Opposite impact on 14C-2-deoxyglucose brain metabolism following patterns of high and low frequency repetitive transcranial magnetic stimulation in the posterior parietal cortex

Repetitive transcranial magnetic stimulation (rTMS) appears capable of modulating human cortical excitability beyond the duration of the stimulation train. However, the basis and extent of this "off-line" modulation remains unknown. In a group of anesthetized cats, we applied patterns of real or sham focal rTMS to the visuo-parietal cortex (VP) at high (HF) or low (LF) frequency and recorded brain glucose uptake during (on-line), immediately after (off-line), or 1 h after (late) stimulation. During the on-line period LF and HF rTMS induced a significant relative reduction of (14)C-2DG uptake in the stimulated VP cortex and tightly linked cortical and subcortical structures (e.g. the superficial superior colliculus, the pulvinar, and the LPl nucleus) with respect to homologue areas in the unstimulated hemisphere. During the off-line period HF rTMS induced a significant relative increase in (14)C-2DG uptake in the targeted VP cortex, whereas LF rTMS generated the opposite effect, with only mild network impact. Moderate distributed effects were only recorded after LF rTMS in the posterior thalamic structures. No long lasting cortical or subcortical effects were detected during the late period. Our findings demonstrate opposite modulation of rTMS on local and distant effects along a specific network, depending on the pattern of stimulation. Such effects are demonstrated in the anesthetized animal, ruling out behavioral and non-specific reasons for the differential impact of the stimulation. The findings are consistent with previous differential electrophysiological and behavioral effects of low and high frequency rTMS patterns and provide support to uses of rTMS in neuromodulation.

Categorical perception of orientation in monkeys

Structural and functional substrates of orientation processing in monkeys have been clarified. However, orientation perception in monkeys has not been fully studied. In this study, the cognitive mechanism that controls monkeys' perception of orientation was evaluated. After the monkeys were trained to discriminate between a cardinal and an oblique orientation (e.g., 0 degrees and 30 degrees), their perceptual mechanisms underlying orientation discrimination were tested by using six orientations, ranging from 0 degrees to 150 degrees, including ones used in the discrimination training. Generalization tests showed that the monkeys who were trained with cardinal orientations (e.g., 0 degrees) as positive stimuli generalized their responses to the other cardinal orientation (e.g., 90 degrees). Similarly, the monkeys who were trained with oblique orientations (e.g., 30 degrees) as positive stimuli generalized their responses to all other oblique orientations (e.g., 60 degrees, 120 degrees, and 150 degrees). These findings indicated that the monkeys abstracted the quality of the cardinal/oblique category from the physical features of orientation stimuli although they were not trained to do so. Such an abstraction also suggested a discrepancy between a continuously and orderly arranged cortical map and a discontinuously categorized perception of orientation. The present findings provide insight into the learning-correlated plasticity of cortical orientation preference.

Intrinsic signal recording from a monkey whose behavior was maintained by a schedule of reinforcement

Optical recording of cortical activity in awake monkeys has enhanced our understanding of the functional anatomy of the primary visual cortex (V1). However, cortical representation of visual cognition has not been studied by optical recording, even though the greatest merit of using awake animals is that they can offer advantages in studying cognitive function that anesthetized animals cannot. Thus far, the optical recording method has not been combined with tasks that accompany body movements because of concern about movement noise, although behavioral tasks are helpful in the study of animal cognition. Here, I tested the influence of body movements during the signal acquisition period on the resultant images. I recorded the intrinsic signals associating with different orientations from V1 of a monkey who was emitting behavior during the signal acquisition period. Although the monkey's behavior was maintained on a variable-interval schedule that typically induces a high rate of response, orientation maps were consistently obtained. Therefore, a successful recording under this operant regimen implies the applicability of the optical recording method to other behavioral tasks. Several constraints in applying optical recording to studies using behaving animals are also discussed.

Visual detection deficits following inactivation of the superior colliculus in the cat

Lesion or inactivation of the superior colliculus (SC) of the cat results in an animal that fails to orient toward peripheral visual stimuli which normally evoke a brisk, reflexive orienting response. A failure to orient toward a visual stimulus could be the result of a sensory impairment (a failure to detect the visual stimulus) or a motor impairment (an inability to generate the orienting response). Either mechanism could explain the deficit observed during SC inactivation since neurons in the SC can carry visual sensory signals as well as motor commands involved in the generation of head and eye movements. We investigated the effects of SC inactivation in the cat in two ways. First, we tested cats in a visual detection task that required the animals to press a central, stationary foot pedal to indicate detection of a peripheral visual stimulus. Such a motor response does not involve any components of the orienting response and is unlikely to depend on SC motor commands. A deficit in this task would indicate that the SC plays an important role in the detection of visual targets even in a task that does not require visual orienting. Second, to further investigate the visual orienting deficit observed during SC inactivation and to make direct comparisons between detection and orienting performance, we tested cats in a standard perimetry paradigm. Performance in both tasks was tested following focal inactivation of the SC with microinjections of muscimol at various depths and rostral/caudal locations throughout the SC. Our results reveal a dramatic deficit in both the visual detection task and the visual orienting task following inactivation of the SC with muscimol.

The attentive homunculus: now you see it, now you don't

The nature of the neural system that directs our attention toward selective items in the extrapersonal world is a longstanding and interesting puzzle. The ability to image the human brain at work non-invasively using positron-emission tomography or functional magnetic resonance has provided the means to investigate this issue. In this article, I review the contributions of brain imaging toward the characterization of attentional control in the human brain. The majority of experiments to date have investigated visual spatial orienting. A consistent pattern of brain areas has been revealed, comprising most notably the posterior parietal cortex around the intraparietal sulcus and frontal regions including the frontal eye fields. The brain areas implicated in the control of visual spatial attention were noted to resemble those involved in the control of eye movements, and direct experimental comparisons supported a tight link between the two systems. The findings suggested a sensible view of the attentional 'homunculus' as a distributed neural system related to the control of eye movements. Eye movements form perhaps the most basic orienting response, and can be shifted rapidly and efficiently based on multiple frames of reference. Some attention experiments using objects and features instead of spatial locations as the target of selection also obtained similar patterns of parietal-frontal activations, rendering further support to this view of the attentional control system. Some recent experiments, however, have cautioned against a premature conclusion regarding the ubiquity of the attentional control system revealed by studies of visual spatial attention. Different parietal and frontal regions become engaged when attention is shifted along non-spatial dimensions, such as when attention is directed toward a particular motor act or toward a specific point in time. In these cases, the neural system resembles those involved in the control of limb movements. The attentional homunculus thus begins to dissolve. The alternative view suggested is that attentional control may be a property of specialized parietal-frontal systems that transform perception into action. Future studies will be needed to validate this view of attention, or to provide a more mature understanding of its true nature.

Learned discrimination of pattern orientation in walking flies

To determine the pattern-orientation discrimination ability of blowflies, Phaenicia sericata, a learning/memory assay was developed in which sucrose served as the reward stimulus and was paired with one of two visual gratings of different orientations. Individual, freely walking flies with clipped wings were trained to discriminate between pairs of visual patterns presented in the vertical plane. During training trials, individual flies learned to search preferentially at the rewarded stimulus. In subsequent testing trials, flies continued to exhibit a learned preference for the previously rewarded stimulus, demonstrating an ability to discriminate between the two visual cues. Flies learned to discriminate between horizontal and vertical gratings, +45 degrees (relative to a 0 degrees vertical) and −45 degrees gratings, and vertical and +5 degrees gratings. Individual patterns of learning and locomotive behavior were observed in the pattern of exploration during training trials. The features of the visual cue critical for discrimination of orientation are discussed.

Global relationship between anatomical connectivity and activity propagation in the cerebral cortex

Anatomical connectivity is a prerequisite for cooperative interactions between cortical areas, but it has yet to be demonstrated that association fibre networks determine the macroscopical flow of activity in the cerebral cortex. To test this notion, we constructed a large-scale model of cortical areas whose interconnections were based on published anatomical data from tracing studies. Using this model we simulated the propagation of activity in response to activation of individual cortical areas and compared the resulting topographic activation patterns to electrophysiological observations on the global spread of epileptic activity following intracortical stimulation. Here we show that a neural network with connectivity derived from experimental data reproduces cortical propagation of activity significantly better than networks with different types of neighbourhood-based connectivity or random connections. Our results indicate that association fibres and their relative connection strengths are useful predictors of global topographic activation patterns in the cerebral cortex. This global structure-function relationship may open a door to explicit interpretation of cortical activation data in terms of underlying anatomical connectivity.

A method to assess the functional impact of cerebral connections on target populations of neurons

We describe an innovative and tested approach combining two individually potent techniques to visualize simultaneously the functional impact of multiple projections on target populations of neurons in the brain. The rationale is simple: silence a defined set of efferent projections from one cortical region using cooling deactivation and then measure the impact of the deactivation on activities in multiple target regions using 2-deoxyglucose (2DG). This is a straightforward and sound approach because 2DG uptake by neurons reflects levels of underlying neural activity. All distant modifications evoked by the silencing of the set of efferent projections are examined in anatomical tissue and simultaneously for the multiple target sites to provide a global view of the functional impacts of the set of projections on the targets. With this method, downward adjustments of 2DG uptake levels identify removals of net excitatory signals, whereas upward adjustments identify net removals of suppressive influences. Future possible uses and modifications of the technique, including optical imaging, are discussed. Overall, the technique has the potential to provide fundamental, new measures on cerebral network interactions that both complement and extend current static models of cerebral networks and electrophysiological measures of functional impacts on individual neurons.

The neuronal machinery involved in successive orientation discrimination

Following our strategy of using simple discrimination tasks to investigate the primate visual system, we trained both human and monkey subjects for two orientation discrimination tasks: an identification and a successive discrimination. Contrasting these two tasks allowed us to isolate the temporal comparison component and to relate this component to activity in right fusiform gyrus using Positron Emission Tomography (PET) and to infero-temporal cortex using a lesion approach in monkeys. Single-cell recordings in infero-temporal cortex demonstrated that neurons in this region can contribute to the three processes underlying temporal comparison: (1) sensorial representation of visual stimuli, (2) maintaining a trace of the preceding stimulus, and (3) comparison of the incoming stimulus with that trace. By the same token, a comparison of these two tasks, which use the same input and the same attribute, demonstrates the task dependency of processing in the human and non-human primate visual system.

Functional impact of cerebral connections

Cerebral networks are complex sets of connections that resemble a ladder-like web of multiple parallel feedforward, lateral, and feedback connections. This static anatomical description has been pivotal in guiding our understanding of signal processing within cerebral networks. However, measures on both magnitude and functional significance of connections are extremely limited. Here, we compare the anatomically defined strengths of a set of cerebral pathways emerging from the visual middle suprasylvian (MS) cortex of the cat with measures of the functional impact the same region has over distant sites. These functional measures were obtained by analyzing the local and distant effects of MS cooling deactivation on deoxyglucose uptake. Relative to major efferent projections from MS cortex that have a strong influence, projections to early visual processing stages have weaker functional influences than predicted from the anatomy. For higher processing stages, the converse holds: projections from MS cortex have stronger functional influence than predicted from the anatomy. We conclude that these and future functional measures, obtained using the same combination of techniques, will furnish fundamental, new information that complements and extends current models of static cerebral networks, and lead to more realistic models of cerebral network function and component interactions.