Analysing a visual scene requires the brain to briefly keep in memory potentially relevant items of that scene and then direct attention to their locations for detailed processing. To reveal the ...neuronal basis of the underlying working memory and top-down attention processes, we trained macaques to match two patterns presented with a delay between them. As the above processes are likely to require communication between brain regions, and the parietal cortex is known to be involved in spatial attention, we simultaneously recorded neuronal activities from the interconnected parietal and middle temporal areas. We found that mnemonic information about features of the first pattern was retained in coherent oscillating activity between the two areas in high-frequency bands, followed by coherent activity in lower frequency bands mediating top-down attention on the relevant spatial location. Oscillations maintaining featural information also modulated activity of the cells of the parietal cortex that mediate attention. This could potentially enable transfer of information to organize top-down signals necessary for selective attention. Our results provide evidence in support of a two-stage model of visual attention where the first stage involves creating a saliency map representing a visual scene and at the second stage attentional feedback is provided to cortical areas involved in detailed analysis of the attended parts of a scene.
We tested the hypothesis that in a cluttered visual scene, the magnocellular (M) pathway is crucial for focusing attention serially on the objects in the field. Since developmental dyslexia is ...commonly associated with an M pathway deficit, we compared reading impaired children and age-matched normal readers in a search task that required the detection of a target defined by the conjunction of two features, namely form and colour, that are processed by the parvocellular dominated ventral neocortical stream. The dyslexic group's performance was significantly poorer than the controls when there were a large number of distractor items. The scheme of selective attention proposed from these results provides a neural mechanism that underlies reading and explains the pathophysiology of dyslexia.
Neuronal responses were recorded from the striate cortex of monkeys trained to perform visual discrimination at locations in the visual field to which their attention was drawn. A subset of neurons ...showed vigorous responses to visual stimuli for trials in which the monkey was directing its attention to the respective receptive field location. In trials where attention is directed elsewhere, responses to the same stimuli were significantly reduced. In some cells the early response component was not modulated by attention, but later components were affected by the locus of attention. The results suggest the operation of a feedback in the paradigm that spotlights a topographically restricted area of V1 for further processing at higher levels.
Highlights ► Many visual cortical cells show complex responses to superimposed grating stimuli. ► The complex responses are not evident in the orientation tuning to single gratings. ► Such neurons ...are located close to pinwheel centres in the cortical orientation map. ► Results point to multiple mechanisms contributing to orientation selectivity. ► Results are consistent with pinwheel regions having greater neuronal plasticity.
Recent studies have reported an attentional feedback that highlights neural responses as early along the visual pathway as the primary visual cortex. Such filtering would help in reducing ...informational overload and in performing serial visual search by directing attention to individual locations in the visual field. The magnocellular (M) and parvocellular (P) subdivisions are two of the major parallel pathways in primate vision that originate in the retina and carry distinctly different types of information. The M pathway, characterized by its high sensitivity to movement and to low contrast stimuli, forms the predominant visual input into the dorsal, parietal stream in the neocortex. The P inputs, characterized by their colour selectivity and higher spatial resolution, are channeled mainly into the ventral, temporal stream. It is proposed that the attentional spotlight originates in the dorsal stream and helps in serially searching the field for conjunction of the relevant target features in the temporal stream, effectively performing a gating function on all visual inputs. This model predicts that a defect limited to the magnocellular or the dorsal pathway can lead to widespread deficits in cognitive abilities, including those functions that are largely based on parvocellular information. For example, the model provides a neural mechanism linking a peripheral defect in the magnocellular pathway to the reading disabilities in dyslexia. Even though there has been strong evidence for a magnocellular deficit in dyslexia, the paradox has been that the cognitive disability seems to be related to P pathway function. The scheme proposed here shows how M input may be vital for controlling sequential attention during reading.
We studied the effects of reversible cooling between 35 and 7 °C on membrane properties and spike generation of cells in slices
of rat visual cortex.
Cooling led to a depolarization of the neurones ...and an increase of the input resistance, thus bringing the cells closer to
spiking threshold. Excitability, measured with intracellular current steps, increased with cooling.
Synaptic stimuli were most efficient in producing spikes at room temperature, but strong stimulation could evoke spikes even
below 10 °C.
Spike width and total area increased with cooling, and spike amplitude was maximal between 12 and 20 °C. Repetitive firing
was enhanced in some cells by cooling to 20â25 °C, but was always suppressed at lower temperatures.
With cooling, passive potassium conductance decreased and the voltage-gated potassium current had a higher activation threshold
and lower amplitude. At the same time, neither passive sodium conductance nor the activation threshold of voltage-dependent
sodium channels changed. Therefore changing the temperature modifies the ratio between potassium and sodium conductances,
and thus alters basic membrane properties.
Data from two cells recorded in slices of cat visual cortex suggest a similar temperature dependence of the membrane properties
of neocortical neurones to that described above in the rat.
These results provide a framework for comparison of the data recorded at different temperatures, but also show the limitations
of extending the conclusions drawn from in vitro data obtained at room temperature to physiological temperatures. Further, when cooling is used as an inactivation tool in vivo , it should be taken into account that the mechanism of inactivation is a depolarization block. Only a region cooled below
10 °C is reliably silenced, but it is always surrounded by a domain of hyperexcitable cells.
For over three decades, the mechanism of orientation selectivity of visual cortical neurones has been hotly debated. While intracortical inhibition has been implicated as playing a vital role, it has ...been difficult to observe it clearly. On the basis of recent findings, we propose a model in which the visual cortex brings together a number of different mechanisms for generating orientation-selective responses. Orientation biases in the thalamo-cortical input fibres provide an initial weak selectivity either directly in the excitatory input or by acting via cortical interneurones. This weak selectivity of postsynaptic potentials is then amplified by voltage-sensitive conductances of the cell membrane and excitatory and inhibitory intracortical circuitry, resulting in the sharp tuning seen in the spike discharges of visual cortical cells.
We have developed a reversible system for performing simultaneous recordings from multiple brain areas of trained macaque monkeys. It consists of a near-circular halo fitted around the head of the ...monkey with 5–10 thin plastic or stainless steel posts that either jut against or are screwed into the skull, respectively. Both methods of implantation of the posts are easily reversible, enabling protracted recordings over many years and training the monkeys in more complex tasks. The former is more useful for shorter periods of recordings (2–4 months) separated by long intervals and the latter for longer periods of recordings at a time (6–12 months). With both systems, essentially the entire scalp is intact, allowing multi-site recordings from a number of dorsal cortical areas, as well as other areas, simultaneously. These recordings are performed through tiny craniotomies of usually less than 2mm diameter, which are fitted with small plastic cones that serve as guide tubes for the microelectrodes. The surgery involved in these procedures is relatively minor compared to classical methods and the implants are also usually free of infections, thus requiring little maintenance of recording chambers.
We studied the effects of reversible cooling on synaptic transmission in slices of rat visual cortex. Cooling had marked monotonic effects on the temporal properties of synaptic transmission. It ...increased the latency of excitatory postsynaptic potentials and prolonged their time-course. Effects were non-monotonic on other properties, such as amplitude of excitatory postsynaptic potentials and generation of spikes. The amplitude of excitatory postsynaptic potentials increased, decreased, or remain unchanged while cooling down to about 20°C, but thereafter it declined gradually in all cells studied. The effect of moderate cooling on spike generation was increased excitability, most probably due to the ease with which a depolarized membrane potential could be brought to spike threshold by a sufficiently strong excitatory postsynaptic potential. Stimuli that were subthreshold above 30°C could readily generate spikes at room temperature. Only at well below 10°C could action potentials be completely suppressed. Paired-pulse facilitation was less at lower temperatures, indicating that synaptic dynamics are different at room temperature as compared with physiological temperatures.
These results have important implications for extrapolating
in vitro data obtained at room temperatures to higher temperatures. The data also emphasize that inactivation by cooling might be a useful tool for studying interactions between brain regions, but the data recorded within the cooled area do not allow reliable conclusions to be drawn about neural operations at normal temperatures.
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