Direction-selective visual responses in macaque superior colliculus induced by behavioral training

Decision-related activity and movement selection in primate visual cortex

Fluctuations in the activity of sensory neurons often predict perceptual decisions. This connection can be quantified with a metric called choice probability (CP), and there is a longstanding debate about whether CP reflects a causal influence on decisions or an echo of decision-making activity elsewhere in the brain. Here, we show that CP can reflect a third variable, namely, the movement used to indicate the decision. In a standard visual motion discrimination task, neurons in the middle temporal (MT) area of primate cortex responded more strongly during trials that involved a saccade toward their receptive fields. This variability accounted for much of the CP observed across the neuronal population, and it arose through training. Moreover, pharmacological inactivation of MT biased behavioral responses away from the corresponding visual field locations. These results demonstrate that training on a task with fixed sensorimotor contingencies introduces movement-related activity in sensory brain regions and that this plasticity can shape the neural circuitry of perceptual decision-making.

A case for human systems neuroscience

Can the human brain itself serve as a model for a systems neuroscience approach to understanding the human brain? After all, how the brain is able to create the richness and complexity of human behavior is still largely mysterious. What better choice to study that complexity than to study it in humans? However, measurements of brain activity typically need to be made non-invasively which puts severe constraints on what can be learned about the internal workings of the brain. Our approach has been to use a combination of psychophysics in which we can use human behavioral flexibility to make quantitative measurements of behavior and link those through computational models to measurements of cortical activity through magnetic resonance imaging. In particular, we have tested various computational hypotheses about what neural mechanisms could account for behavioral enhancement with spatial attention (Pestilli et al., 2011). Resting both on quantitative measurements and considerations of what is known through animal models, we concluded that weighting of sensory signals by the magnitude of their response is a neural mechanism for efficient selection of sensory signals and consequent improvements in behavioral performance with attention. While animal models have many technical advantages over studying the brain in humans, we believe that human systems neuroscience should endeavor to validate, replicate and extend basic knowledge learned from animal model systems and thus form a bridge to understanding how the brain creates the complex and rich cognitive capacities of humans.

Organization of the gymnotiform fish pallium in relation to learning and memory: II. Extrinsic connections

This study describes the extrinsic connections of the dorsal telencephalon (pallium) of gymnotiform fish. We show that the afferents to the dorsolateral and dorsomedial pallial subdivisions of gymnotiform fish arise from the preglomerular complex. The preglomerular complex receives input from four clearly distinct regions: (1) descending input from the pallium itself (dorsomedial and dorsocentral subdivisions and nucleus taenia); (2) other diencephalic nuclei (centroposterior, glomerular, and anterior tuberal nuclei and nucleus of the posterior tuberculum); (3) mesencephalic sensory structures (optic tectum, dorsal and ventral torus semicircularis); and (4) basal forebrain, preoptic area, and hypothalamic nuclei. Previous studies have implicated the majority of the diencephalic and mesencephalic nuclei in electrosensory, visual, and acousticolateral functions. Here we discuss the implications of preglomerular/pallial electrosensory-associated afferents with respect to a major functional dichotomy of the electric sense. The results allow us to hypothesize that a functional distinction between electrocommunication vs. electrolocation is maintained within the input and output pathways of the gymnotiform pallium. Electrocommunication information is conveyed to the pallium through complex indirect pathways that originate in the nucleus electrosensorius, whereas electrolocation processing follows a conservative pathway inherent to all vertebrates, through the optic tectum. We hypothesize that cells responsive to communication signals do not converge onto the same targets in the preglomerular complex as cells responsive to moving objects. We also hypothesize that efferents from the dorsocentral (DC) telencephalon project to the dorsal torus semicircularis to regulate processing of electrocommunication signals, whereas DC efferents to the tectum modulate sensory control of movement.

Signal multiplexing and single-neuron computations in lateral intraparietal area during decision-making

Previous work has revealed a remarkably direct neural correlate of decisions in the lateral intraparietal area (LIP). Specifically, firing rate has been observed to ramp up or down in a manner resembling the accumulation of evidence for a perceptual decision reported by making a saccade into (or away from) the neuron's response field (RF). However, this link between LIP response and decision formation emerged from studies where a saccadic target was always stimulating the RF during decisions, and where the neural correlate was the averaged activity of a restricted sample of neurons. Because LIP cells are (1) highly responsive to the presence of a visual stimulus in the RF, (2) heterogeneous, and (3) not clearly anatomically segregated from large numbers of neurons that fail selection criteria, the underlying neuronal computations are potentially obscured. To address this, we recorded single neuron spiking activity in LIP during a well-studied moving-dot direction-discrimination task and manipulated whether a saccade target was present in the RF during decision-making. We also recorded from a broad sample of LIP neurons, including ones conventionally excluded in prior studies. Our results show that cells multiplex decision signals with decision-irrelevant visual signals. We also observed disparate, repeating response "motifs" across neurons that, when averaged together, resemble traditional ramping decision signals. In sum, neural responses in LIP simultaneously carry decision signals and decision-irrelevant sensory signals while exhibiting diverse dynamics that reveal a broader range of neural computations than previously entertained.

Is saccade averaging determined by visual processing or movement planning?

Saccadic averaging that causes subjects' gaze to land between the location of two targets when faced with simultaneously or sequentially presented stimuli has been often used as a probe to investigate the nature of computations that transform sensory representations into an oculomotor plan. Since saccadic movements involve at least two processing stages-a visual stage that selects a target and a movement stage that prepares the response-saccade averaging can either occur due to interference in visual processing or movement planning. By having human subjects perform two versions of a saccadic double-step task, in which the stimuli remained the same, but different instructions were provided (REDIRECT gaze to the later-appearing target vs. FOLLOW the sequence of targets in their order of appearance), we tested two alternative hypotheses. If saccade averaging were due to visual processing alone, the pattern of saccade averaging is expected to remain the same across task conditions. However, whereas subjects produced averaged saccades between two targets in the FOLLOW condition, they produced hypometric saccades in the direction of the initial target in the REDIRECT condition, suggesting that the interaction between competing movement plans produces saccade averaging.

Colour and pattern selectivity of receptive fields in superior colliculus of marmoset monkeys

The main subcortical visual targets of retinal output neurones (ganglion cells) are the parvocellular and magnocellular layers of the dorsal lateral geniculate nucleus (LGN) in the thalamus. In addition, a small and heterogeneous collection of ganglion cell axons projects to the koniocellular layers of the LGN, to the superior colliculus (SC), and to other subcortical targets. The functional (receptive field) properties and target specificity of these non-parvocellular, non-magnocellular populations remain poorly understood. It is known that one population of koniocellular layer cells in the LGN (blue-On cells) receives dominant functional input from short-wavelength sensitive (S or ‘blue') cones. Here we asked whether SC neurones also receive S cone inputs. We made extracellular recordings from single neurones (n = 38) in the SC of anaesthetised marmoset monkeys. Responses to drifting and flashed gratings providing defined levels of cone contrast were measured. The SC receptive fields we recorded were often binocular, showed ‘complex cell' like responses (On–Off responses), strong bandpass spatial frequency tuning, direction selectivity, and many showed strong and rapid habituation to repeatedly presented stimuli. We found no evidence for dominant S cone input to any SC neurone recorded. These data suggest that S cone signals may reach cortical pathways for colour vision exclusively through the koniocellular division of the lateral geniculate nucleus.

Neuronal responses to moving targets in monkey frontal eye fields

Due to delays in visuomotor processing, eye movements directed toward moving targets must integrate both target position and velocity to be accurate. It is unknown where and how target velocity information is incorporated into the planning of rapid (saccadic) eye movements. We recorded the activity of neurons in frontal eye fields (FEFs) while monkeys made saccades to stationary and moving targets. A substantial fraction of FEF neurons was found to encode not only the initial position of a moving target, but the metrics (amplitude and direction) of the saccade needed to intercept the target. Many neurons also encoded target velocity in a nearly linear manner. The quasi-linear dependence of firing rate on target velocity means that the neuronal response can be directly read out to compute the future position of a target moving with constant velocity. This is demonstrated using a quantitative model in which saccade amplitude is encoded in the population response of neurons tuned to retinal target position and modulated by target velocity.

Temporal dynamics of decision-making during motion perception in the visual cortex

How does the brain make decisions? Speed and accuracy of perceptual decisions covary with certainty in the input, and correlate with the rate of evidence accumulation in parietal and frontal cortical "decision neurons". A biophysically realistic model of interactions within and between Retina/LGN and cortical areas V1, MT, MST, and LIP, gated by basal ganglia, simulates dynamic properties of decision-making in response to ambiguous visual motion stimuli used by Newsome, Shadlen, and colleagues in their neurophysiological experiments. The model clarifies how brain circuits that solve the aperture problem interact with a recurrent competitive network with self-normalizing choice properties to carry out probabilistic decisions in real time. Some scientists claim that perception and decision-making can be described using Bayesian inference or related general statistical ideas, that estimate the optimal interpretation of the stimulus given priors and likelihoods. However, such concepts do not propose the neocortical mechanisms that enable perception, and make decisions. The present model explains behavioral and neurophysiological decision-making data without an appeal to Bayesian concepts and, unlike other existing models of these data, generates perceptual representations and choice dynamics in response to the experimental visual stimuli. Quantitative model simulations include the time course of LIP neuronal dynamics, as well as behavioral accuracy and reaction time properties, during both correct and error trials at different levels of input ambiguity in both fixed duration and reaction time tasks. Model MT/MST interactions compute the global direction of random dot motion stimuli, while model LIP computes the stochastic perceptual decision that leads to a saccadic eye movement.

Multisensory Versus Unisensory Integration: Contrasting Modes in the Superior Colliculus

The present study suggests that the neural computations used to integrate information from different senses are distinct from those used to integrate information from within the same sense. Using superior colliculus neurons as a model, it was found that multisensory integration of cross-modal stimulus combinations yielded responses that were significantly greater than those evoked by the best component stimulus. In contrast, unisensory integration of within-modal stimulus pairs yielded responses that were similar to or less than those evoked by the best component stimulus. This difference is exemplified by the disproportionate representations of superadditive responses during multisensory integration and the predominance of subadditive responses during unisensory integration. These observations suggest that different rules have evolved for integrating sensory information, one (unisensory) reflecting the inherent characteristics of the individual sense and, the other (multisensory), unique supramodal characteristics designed to enhance the salience of the initiating event.

Impact of Experience on the Representation of Object-Centered Space in the Macaque Supplementary Eye Field

Many neurons in the macaque supplementary eye field (SEF) exhibit object-centered spatial selectivity, firing at different rates when the monkey plans a saccade to the left or right end of a horizontal bar. Is this property natural to the SEF or is it a product of specialized training in the laboratory? To answer this question, we monitored the activity of single SEF neurons in two monkeys before and after training to select eye-movement targets by an object-centered rule. During stage 1, the monkeys performed a color delayed-match-to-sample (DMS) task in which a red or green central cue dictated an eye movement to the matching end of a horizontal bar. Many neurons at this stage exhibited object-centered spatial selectivity. During stage 2, the monkeys performed a color-conditional object-centered task in which a green or red central cue instructed an eye movement to the left or right end of a gray bar. More neurons exhibited object-centered spatial selectivity during this stage than during stage 1. During stage 3, the monkeys again performed the color DMS task. The fraction of neurons exhibiting object-centered spatial selectivity remained at a level comparable to that observed during stage 2 and above that observed during stage 1. Thus object-centered spatial selectivity was present before training on an object-centered rule, was enhanced as a product of object-centered training, and outlasted active use of an object-centered rule. We conclude that neural representations of object-centered space, naturally present in the primate brain, can be sharpened by training.

Hippocalcin-deficient mice display a defect in cAMP response element-binding protein activation associated with impaired spatial and associative memory

Hippocalcin is a member of the neuronal calcium sensor (NCS) protein family that is highly expressed in hippocampal pyramidal cells and moderately expressed in the neurons of cerebral cortex, cerebellum and striatum. Here we examined the physiological roles of hippocalcin using targeted gene disruption. Hippocalcin-deficient (-/-) mice displayed no obvious structural abnormalities in the brain including hippocampal formation at the light microscopic level. Deletion of hippocalcin did not result in up-regulation of the hippocalcin-related proteins; neural visinin-like Ca(2+)-binding proteins (NVP) 1, 2, and 3. The synaptic excitability of hippocampal CA1 neurons appeared to be normal, as estimated by the shape of field excitatory postsynaptic potentials elicited by single- and paired-pulse stimuli, and by tetanic stimulation. However, N-methyl-d-aspartate stimulation- and depolarization-induced phosphorylation of cAMP-response element-binding protein (CREB) was significantly attenuated in -/- hippocampal neurons, suggesting an impairment in an activity-dependent gene expression cascade. In the Morris water maze test, the performance of -/- mice was comparable to that of wild-type littermates except in the probe test, where -/- mice crossed the previous location of the platform significantly less often than +/+ mice. Hippocalcin-deficient mice were also impaired on a discrimination learning task in which they needed to respond to a lamp illuminated on the left or right side to obtain food reinforcement. No abnormalities were observed in motor activity, anxiety behavior, or fear learning. These results suggest that hippocalcin plays a crucial role in the Ca(2+)-signaling pathway that underlies long-lasting neural plasticity and that leads to spatial and associative memory.