We have documented previously a close relationship between neuronal activity in the middle temporal visual area (MT or V5) and behavioral judgments of motion (Newsome et al., 1989; Salzman et al., 1990; Britten et al., 1992; Britten et al., 1996). We have now used numerical simulations to try to understand how neural signals in area MT support psychophysical decisions. We developed a model that pools neuronal responses drawn from our physiological data set and compares average responses in different pools to produce psychophysical decisions. The structure of the model allows us to assess the relationship between “neuronal ” input signals and simulated psychophysical performance using the same methods we have applied to real experimental data. We sought to reconcile three experimental observations: psychophysical performance (threshold sensitivity to motion

Tufted layer 5 (TL5) pyramidal neurons are important projection neurons from the cerebral cortex to subcortical areas. Recent and ongoing experiments aimed at understanding the computational analysis performed by a network of synaptically connected TL5 neurons are reviewed here. The experiments employed dual and triple whole-cell patch clamp recordings from visually identified and preselected neurons in brain slices of somatosensory cortex of young (14- to 16-day-old) rats. These studies suggest that a local network of TL5 neurons within a cortical module of diameter 300 μm consists of a few hundred neurons that are extensively interconnected with reciprocal feedback from at least first-, second- and third-order target neurons. A statistical analysis of synaptic innervation suggests that this recurrent network is not randomly arranged and hence each neuron could be functionally unique. Synaptic transmission between these neurons is characterized by use-dependent synaptic depression which confers novel properties to this recurrent network of neurons. First, a range of rates of depression for different synaptic connections enable each TL5 neuron to receive a unique mixture of information about the average firing rates and the temporally correlated action potential (AP) activity in the population of presynaptic TL5 neurons. Second, each AP generated by any neuron in the network induces a change (defined as an iteration step) in the functional coupling of the neurons in the network (defined as network configuration). It is proposed that the network configuration is iterated during a stimulus to achieve an optimally orchestrated network response. Hebbian, anti-Hebbian and neuromodulatory-induced modifications of neurotransmitter release probability change the rates of synaptic depression and thereby alter the iteration step size. These data may be important to understand the dynamics of electrical activity within the network.

We develop a mathematical formalism for calculating connectivity volumes generated by specific topologies with various physical packing strategies. We consider four topologies (full, random, nearest neighbor, and modular connectivity) and three physical models: (i) interior packing, where neurons and connection fibers are intermixed, (ii) sheeted packing where neurons are located on a sheet with fibers running underneath, and (iii) exterior packing where the neurons are located at the surfaces of a cube or sphere with fibers taking up the internal volume. By extensive cross-referencing of available human neuroanatomical data we produce a consistent set of parameters for the whole brain, the cerebral cortex, and the cerebellar cortex. By comparing these inferred values with those predicted by the expressions, we draw the following general conclusions for the human brain, cortex, cerebellum: (i) Interior packing is less efficient than exterior packing (in a sphere). (ii) Fully and rando...

this paper show that a distributed network of visual areas in the monkey contains information about the direction of motion of a stimulus, in agreement with previous single-unit electrophysiology studies. Specifically, the BOLD signal in areas V1, V2, V3, V3A, V4, and MT reflects the processing Figure 5. Information about the direction of motion in areas V2, V3, V3A, and V4. Lef t column shows the area of interest, marked in yellow,on a single horizontal slice of an individual monkey during one experimental session. Right column (same notations as in Fig. 4) shows the average activities of the different visual areas. This activity is the mean across all significant voxels in all slices, monkeys, and experimental sessions belonging to a particular visual area. Histogram insets have the same notation as in Figure 4. Voxels belonging to area V2 were identified within the posterior bank of the lunate sulcus (ls) excluding the f undus. The time course of the mean BOLD activity was computed from a total of 157 voxels, six slices, three monkeys, and five experimental sessions. Voxels belonging to area V2/V3 were typically identified within the inferior occipital sulcus (ios). The time course of the mean BOLD activity was computed from a total of 680 voxels, six slices, four monkeys, and four experimental sessions. Voxels belonging to area V3A were identified 4 within the anterior bank of the ls, excluding the f undus. The time course of the mean BOLD activity was computed from a total of 54 voxels, five slices, four monkeys, and four experimental sessions. Voxels belonging to V3/V3A were identified within the anterior bank of the ls, including the f undus. The time course of the mean BOLD activity was computed from a total of 170 voxels, five slices, four monkeys, and four e...