Marek Bekisz

Two Streams of Attention-Dependent β Activity in the Striate Recipient Zone of Cat's Lateral Posterior–Pulvinar Complex

Local field potentials from different visual cortical areas and subdivisions of the cats lateral posterior–pulvinar complex of the thalamus (LP-P) were recorded during a behavioral task based on delayed spatial discrimination of visual or auditory stimuli. During visual but not auditory attentive tasks, we observed an increase of β activity (12–25 Hz) as calculated from signals recorded from the caudal part of the lateral zone of the LP-P (LPl-c) as well as from cortical areas 17 and 18 and the complex located at the middle suprasylvian sulcus (MSS). This β activity appeared only in the trials that ended with a successful response, proving its relationship to the mechanism of visual attention. In contrast, no enhanced β activity was observed in the rostral part of the lateral zone of the LP-P and in the pulvinar proper. Two subregions of LPl-c (ventromedial and dorsolateral) were distinguished by visually related, attentional β activity of low (12–18 Hz) and high (18–25 Hz) frequencies, respectively. At the same time, area 17 exhibited attentional activation in the whole β range, and an increase of power in low-frequency β was observed in the medial bank of MSS, whereas cortical area 18 and the lateral bank of the MSS were activated in the high β range. Phase-correlation analysis revealed that two distinct corticothalamic systems were synchronized by the β activity of different frequencies. One comprised of cortical area 17, ventromedial region of LPl-c, and medial MSS, the second involved area 18 and the dorsolateral LPl-c. Our observations suggest that LPl-c belongs to the wide corticothalamic attentional system, which is functionally segregated by distinct streams of β activity.

Attention-dependent coupling between beta activities recorded in the cat's thalamic and cortical representations of the central visual field

We have previously proposed that enhanced 16–24 Hz (beta) local field potential activity in the primary visual cortex and lateral geniculate nucleus may be an electrophysiological correlate of the attentional mechanism that increases the gain of afferent visual information flow to the cortex. In this study, we measured coupling between beta signals recorded in the thalamic (i.e. lateral geniculate or perigeniculate) and cortical representations of the central visual field (within 5° from area centralis), during visual and auditory attentive situations. Signal coupling was calculated in two ways: (i) by means of crosscorrelation between raw beta activities, which depends primarily on phase coherence, and (ii) by phase‐independent crosscorrelation between amplitude envelopes of beta activities. Mean amplitudes of raw signal crosscorrelations obtained for thalamo‐cortical recording pairs were not significantly different when calculated during behavioural demands for either visual or auditory attention. In contrast, amplitudes of envelope crosscorrelations obtained during behaviour requiring visual attention were, on average, two times higher than those calculated during the auditory task. This attention‐related coupling emerged from synchronized amplitude modulation of beta oscillatory activity that occurs within the cortico‐thalamic circuit involved in central vision.

Coupling of beta and gamma activity in corticothalamic system of cats attending to visual stimuli.

We have previously shown that local field potentials (LFPs) recorded in the lateral geniculate nucleus (LGN) and primary visual cortex (VCx) of the cat contain significantly more beta frequency activity when the animal attends to visual than to auditory stimuli. In the present study we utilised data from the same experiments to calculate the cross-correlation between envelopes of filtered beta (16-24Hz) and gamma (30-45Hz) oscillatory signals recorded at the same sites. Correlation values obtained from visual trials were significantly higher than those calculated for the auditory task. This observation was typical in those LGN and VCx recording sites which corresponded to the central representation of the visual field. The cross-correlation function peaked within a 20 ms time window from the centre of the cross-correlogram. These findings support the hypothesis that beta activity provides an excitatory background for the appearance of oscillations in the gamma band.

Increased excitability of cortical neurons induced by associative learning: an ex vivo study.

In adult mice, classical conditioning in which whisker stimulation is paired with an electric shock to the tail results in a decrease in the frequency of head movements, induces expansion of the cortical representation of stimulated vibrissae and enhances inhibitory synaptic interactions within the ‘trained’ barrels. We investigated whether such a simple associative learning paradigm also induced changes in neuronal excitability. Using whole‐cell recordings from ex vivo slices of the barrel cortex we found that layer IV excitatory cells located in the cortical representation of the ‘trained’ row of vibrissae had a higher frequency of spikes recorded at threshold potential than neurons from the ‘untrained’ row and than cells from control animals. Additionally, excitatory cells within the ‘trained’ barrels were characterized by increased gain of the input–output function, lower amplitudes of fast after‐hyperpolarization and decreased effect of blocking of BK channels by iberiotoxin. These findings provide new insight into the possible mechanism for enhanced intrinsic excitability of layer IV excitatory neurons. In contrast, the fast spiking inhibitory cells recorded in the same barrels did not change their intrinsic excitability after the conditioning procedure. The increased excitability of excitatory neurons within the ‘trained’ barrels may represent the counterpart of homeostatic plasticity, which parallels enhanced synaptic inhibition described previously. Together, the two mechanisms would contribute to increase the input selectivity within the conditioned cortical network.

High affinity carnitine transporters from OCTN family in neural cells.

Neurons are known to accumulate l-carnitine—a compound necessary for transfer of acyl moieties through biological membranes, apart from very low β-oxidation of fatty acids in adult brain. Present study demonstrates expression of octn2 and octn3 genes coding high affinity carnitine transporters, as well as presence of both proteins in neurons obtained from suckling and adult rats, and also in mouse transformed neural cells. Measurements of carnitine transport show activity of both transporters in neural cells, pointing to their importance in physiological processes other than β-oxidation.

Visual classification of X and Y perigeniculate neurons of the cat

The spike activity of perigeniculate cells evoked by small light spots flashing along the axes of their receptive fields was recorded and presented in response planes. This method allowed the investigated neurons to be grouped into two classes characterized by (1) large receptive fields and phasic-like responses and (2) small fields and tonic responses. The latency measurements for stimulation of the optic chiasma and visual cortex revealed that the cells from the first group are excited by fast, Y fibers and the second by slow, X axons. The spatial tuning curves of the second harmonic component, as measured from the responses of the cells from the two groups for slowly moving square gratings, are also different. We conclude that the X and Y systems of the visual pathway are segregated at the level of the perigeniculate nucleus.

CD44: a novel synaptic cell adhesion molecule regulating structural and functional plasticity of dendritic spines

CD44 is a novel molecular player that regulates structure and function of the synapse. It affects excitatory synaptic transmission, dendritic spine shape, number of functional synapses, and activity-dependent neuronal plasticity. These functions are exerted via the regulation of small Rho GTPases.

20 Hz Bursts of Activity in the Cortico-Thalamic Pathway During Attentive Perception

It is well established that thalamo-cortical fibers of the visual pathway of the cat send out collaterals to two recurrent loops: inhibitory, via GABAergic interneurons of the perigeniculate nucleus (PGN) and excitatory, relayed by the pyramidal cells of layer 6 of the striate cortex. Both of these loops terminate on principal cells of lateral geniculate nucleus (LGN). There are data indicating the possible role of PGN neurons in synchronization of thalamo-cortical rhythmic activity (see Steriade and Llinas, 1988 for a review), whereas hypotheses concerning the functions of the cortico-geniculate pathway lack clear experimental support. This pathway should be important in view of the fact that cortical axons outnumber all other excitatory inputs to LGN principal cells (Wilson et al., 1984). One of the main reasons for the lack of understanding of the role of cortical input may be the poor responsiveness of layer 6 pyramidal cells in anaesthetized cats. This was demonstrated by Livingstone and Hubel (1981) who showed further that when layer 6 cells become active, after the cat recovered from the anaesthesia, also their specific responses to visual stimuli were noticeably enhanced. These effects were also accompanied by more vigorous responses of LGN principal cells to specific stimulation.

Intraspinal Grafting of Serotonergic Neurons Modifies Expression of Genes Important for Functional Recovery in Paraplegic Rats

Serotonin (5-hydroxytryptamine; 5-HT) plays an important role in control of locomotion, partly through direct effects on motoneurons. Spinal cord complete transection (SCI) results in changes in 5-HT receptors on motoneurons that influence functional recovery. Activation of 5-HT2A and 5-HT7 receptors improves locomotor hindlimb movements in paraplegic rats. Here, we analyzed the mRNA of 5-HT2A and 5-HT7 receptors (encoded by Htr2a and Htr7 genes, resp.) in motoneurons innervating tibialis anterior (TA) and gastrocnemius lateralis (GM) hindlimb muscles and the tail extensor caudae medialis (ECM) muscle in intact as well as spinal rats. Moreover, the effect of intraspinal grafting of serotonergic neurons on Htr2a and Htr7 gene expression was examined to test the possibility that the graft origin 5-HT innervation in the spinal cord of paraplegic rats could reverse changes in gene expression induced by SCI. Our results indicate that SCI at the thoracic level leads to changes in Htr2a and Htr7 gene expression, whereas transplantation of embryonic serotonergic neurons modifies these changes in motoneurons innervating hindlimb muscles but not those innervating tail muscles. This suggests that the upregulation of genes critical for locomotor recovery, resulting in limb motoneuron plasticity, might account for the improved locomotion in grafted animals.

The Primary Visual Cortex Is Differentially Modulated by Stimulus-Driven and Top-Down Attention

Selective attention can be focused either volitionally, by top-down signals derived from task demands, or automatically, by bottom-up signals from salient stimuli. Because the brain mechanisms that underlie these two attention processes are poorly understood, we recorded local field potentials (LFPs) from primary visual cortical areas of cats as they performed stimulus-driven and anticipatory discrimination tasks. Consistent with our previous observations, in both tasks, we found enhanced beta activity, which we have postulated may serve as an attention carrier. We characterized the functional organization of task-related beta activity by (i) cortical responses (EPs) evoked by electrical stimulation of the optic chiasm and (ii) intracortical LFP correlations. During the anticipatory task, peripheral stimulation that was preceded by high-amplitude beta oscillations evoked large-amplitude EPs compared with EPs that followed low-amplitude beta. In contrast, during the stimulus-driven task, cortical EPs preceded by high-amplitude beta oscillations were, on average, smaller than those preceded by low-amplitude beta. Analysis of the correlations between the different recording sites revealed that beta activation maps were heterogeneous during the bottom-up task and homogeneous for the top-down task. We conclude that bottom-up attention activates cortical visual areas in a mosaic-like pattern, whereas top-down attentional modulation results in spatially homogeneous excitation.