Ulf T. Eysel

Neural structures associated with recognition of facial expressions of basic emotions

People with Huntingtons disease and people suffering from obsessive–compulsive disorder show severe deficits in recognizing facial expressions of disgust, whereas people with lesions restricted to the amygdala are especially impaired in recognizing facial expressions of fear. This double dissociation implies that recognition of certain basic emotions may be associated with distinct and non–overlapping neural substrates. Some authors, however, emphasize the general importance of the ventral parts of the frontal cortex in emotion recognition, regardless of the emotion being recognized. In this study, we used functional magnetic resonance imaging to locate neural structures that are critical for recognition of facial expressions of basic emotions by investigating cerebral activation of six healthy adults performing a gender discrimination task on images of faces expressing disgust, fear and anger. Activation in response to these faces was compared with that for faces showing neutral expressions. Disgusted facial expressions activated the right putamen and the left insula cortex, whereas enhanced activity in the posterior part of the right gyrus cinguli and the medial temporal gyrus of the left hemisphere was observed during processing of angry faces. Fearful expressions activated the right fusiform gyrus and the left dorsolateral frontal cortex. For all three emotions investigated, we also found activation of the inferior part of the left frontal cortex (Brodmann area 47). These results support the hypotheses derived from neuropsychological findings, that (i) recognition of disgust, fear and anger is based on separate neural systems, and that (ii) the output of these systems converges on frontal regions for further information processing.

Massive restructuring of neuronal circuits during functional reorganization of adult visual cortex

The cerebral cortex has the ability to adapt to altered sensory inputs. In the visual cortex, a small lesion to the retina causes the deprived cortical region to become responsive to adjacent parts of the visual field. This extensive topographic remapping is assumed to be mediated by the rewiring of intracortical connections, but the dynamics of this reorganization process remain unknown. We used repeated intrinsic signal and two-photon imaging to monitor functional and structural alterations in adult mouse visual cortex over a period of months following a retinal lesion. The rate at which dendritic spines were lost and gained increased threefold after a small retinal lesion, leading to an almost complete replacement of spines in the deafferented cortex within 2 months. Because this massive remodeling of synaptic structures did not occur when all visual input was removed, it likely reflects the activity-dependent establishment of new cortical circuits that serve the recovery of visual responses.

Effect of transcranial magnetic stimulation on single-unit activity in the cat primary visual cortex

Transcranial magnetic stimulation (TMS) has become a well established procedure for testing and modulating the neuronal excitability of human brain areas, but relatively little is known about the cellular processes induced by this rather coarse stimulus. In a first attempt, we performed extracellular single‐unit recordings in the primary visual cortex (area 17) of the anaesthetised and paralysed cat, with the stimulating magnetic field centred at the recording site (2 × 70 mm figure‐of‐eight coil). The effect of single biphasic TMS pulses, which induce a lateral‐to‐medial electric current within the occipital pole of the right hemisphere, was tested for spontaneous as well as visually evoked activity. For cat visual cortex we found that a single TMS pulse elicited distinct episodes of enhanced and suppressed activity: in general, a facilitation of activity was found during the first 500 ms, followed thereafter by a suppression of activity lasting up to a few seconds. Strong stimuli exceeding 50 % of maximal stimulator output could also lead to an early suppression of activity during the first 100–200 ms, followed by stronger (rebound) facilitation. Early suppression and facilitation of activity may be related to a more or less direct stimulation of inhibitory and excitatory interneurons, probably with different thresholds. The late, long‐lasting suppression is more likely to be related to metabotropic or metabolic processes, or even vascular responses. The time course of facilitation/inhibition may provide clues regarding the action of repetitive TMS application.

State-dependent receptive-field restructuring in the visual cortex.

To extract important information from the environment on a useful timescale, the visual system must be able to adapt rapidly to constantly changing scenes. This requires dynamic control of visual resolution, possibly at the level of the responses of single neurons. Individual cells in the visual cortex respond to light stimuli on particular locations (receptive fields) on the retina, and the structure of these receptive fields can change in different contexts. Here we show experimentally that the shape of receptive fields in the primary visual cortex of anaesthetized cats undergoes significant modifications, which are correlated with the general state of the brain as assessed by electroencephalography: receptive fields are wider during synchronized states and smaller during non-synchronized states. We also show that cortical receptive fields shrink over time when stimulated with flashing light spots. Finally, by using a network model we account for the changing size of the cortical receptive fields by dynamically rescaling the levels of excitation and inhibition in the visual thalamus and cortex. The observed dynamic changes in the sizes of the cortical receptive field could be a reflection of a process that adapts the spatial resolution within the primary visual pathway to different states of excitability.

Cellular organization of reciprocal patchy networks in layer III of cat visual cortex (area 17)

There is no direct information available concerning the exact spatial characteristics of long-range axons and their relationship with the patchy phenomena observed after extracellular injection of retrograde tracers. In the present study, using the recently introduced neuronal tracer biocytin, we demonstrate by detailed three-dimensional reconstruction of 10 pyramidal cells in layer III, that their clustered axonal terminals form a specific patchy network in layers II and III. The reconstructed network occupied an area of 6.5 x 3.5 mm parallel to the cortical surface elongated in an anteroposterior direction. The average centre-to-centre distance between patches within the network was 1.1 mm. On average, the axonal field of each of the 10 pyramidal cells contained a total of 417 boutons at four to eight distinct sites (patches), and in each patch, an average of 79 boutons was provided by the same cell. The identified connections between the patches were predominantly reciprocal. Detailed analyses have shown that many pyramidal cells of the network are directly interconnected so that each of them can receive one to four, chiefly axospinous, contacts onto the distal segment of its apical and basal dendrites from the axon of another pyramidal cell belonging to a different patch labelled from the same injection site. We hypothesize that the possible functional role of the network is to link remote sites with similar physiological characteristics, such as orientation preference, supporting the model of Mitchison and Crick [(1982) Proc. natn. Acad. Sci. U.S.A. 79, 3661-3665].

Loss of Sensory Input Causes Rapid Structural Changes of Inhibitory Neurons in Adult Mouse Visual Cortex

A fundamental property of neuronal circuits is the ability to adapt to altered sensory inputs. It is well established that the functional synaptic changes underlying this adaptation are reflected by structural modifications in excitatory neurons. In contrast, the degree to which structural plasticity in inhibitory neurons accompanies functional changes is less clear. Here, we use two-photon imaging to monitor the fine structure of inhibitory neurons in mouse visual cortex after deprivation induced by retinal lesions. We find that a subset of inhibitory neurons carry dendritic spines, which form glutamatergic synapses. Removal of visual input correlates with a rapid and lasting reduction in the number of inhibitory cell spines. Similar to the effects seen for dendritic spines, the number of inhibitory neuron boutons dropped sharply after retinal lesions. Together, these data suggest that structural changes in inhibitory neurons may precede structural changes in excitatory circuitry, which ultimately result in functional adaptation following sensory deprivation.

Membrane properties and spike generation in rat visual cortical cells during reversible cooling

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 20–25 °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.

Evidence for a contribution of lateral inhibition to orientation tuning and direction selectivity in cat visual cortex: reversible inactivation of functionally characterized sites combined with neuroanatomical tracing techniques

We have previously reported that cells in cat areas 17 and 18 can show increases in response to non‐optimal orientations or directions, commensurate with a loss of inhibition, during inactivation of laterally remote, visuotopically corresponding sites by iontophoresis of γ‐aminobutyric acid (GABA). We now present anatomical evidence for inhibitory projections from inactivation sites to recording sites where ‘disinhibitory’ effects were elicited. We made microinjections of [3H]‐nipecotic acid, which selectively exploits the GABA re‐uptake mechanism, < 100 μm from recording sites where cells had shown either an increase in response to non‐optimal orientations during inactivation of a cross‐orientation site (n = 2) or an increase in response to the non‐preferred direction during inactivation of an iso‐orientation site with opposite direction preference (n = 5). Retrogradely labelled GABAergic neurons were detected autoradiographically and their distribution was reconstructed from series of horizontal sections. In every case, radiolabelled cells were found in the vicinity of the inactivation site (three to six within 150 μm). The injection and inactivation sites were located in layers II/III–IV and their horizontal separation ranged from 400 to 560 μm. In another experiment, iontophoresis of biocytin at an inactivation site in layer III labelled two large basket cells with terminals in close proximity to cross‐orientation recording sites in layers II/III where disinhibitory effects on orientation tuning had been elicited. We argue that the inactivation of inhibitory projections from inactivation to recording sites made a major contribution to the observed effects by reducing the strength of inhibition during non‐optimal stimulation in recurrently connected excitatory neurons presynaptic to a recorded cell. The results provide further evidence that cortical orientation tuning and direction selectivity are sharpened, respectively, by cross‐orientation inhibition and iso‐orientation inhibition between cells with opposite direction preferences.

Relationship Between Lateral Inhibitory Connections and the Topography of the Orientation Map in Cat Visual Cortex

The functional and structural topography of lateral inhibitory connections was investigated in visual cortical area 18 using a combination of optical imaging and anatomical tracing techniques in the same tissue. Orientation maps were obtained by recording intrinsic signals in regions of 8.4–19 mm2. To reveal the inhibitory connections provided by large basket cells, biocytin was iontophoretically injected at identified orientation sites guided by the pattern of surface blood vessels. The axonal and dendritic fields of two retrogradely labelled large basket cells were reconstructed in layer III. Their axonal fields extended up to 1360 μm from the parent somata. In addition to single basket cells, the population of labelled basket cell axons was also studied. For this analysis anterogradely labelled basket axons running horizontally over 460–1280 μm from the core of an injection site in layer III were taken into account. The distribution of large basket cell terminals according to orientation preferences of their target regions was quantitatively assessed. Using the same spatial resolution as the orientation map, a frequency distribution of basket cell terminals dependent on orientation specificity could be derived. For individual basket cells, the results showed that, on average, 43% of the terminals provided input to sites showing similar orientation preferences (±30°) to those of the parent somata. About 35% of the terminals were directed to sites representing oblique‐orientation [±(30–60)°], and 22% of them terminated at cross‐orientation sites [±(60–90)°]. Furthermore, the possible impact of large basket cells on target cells at different distances and orientation preferences was estimated by comparing the occurrence of orientation preferences with the occurrence of basket terminals on the distance scale. It was found that a basket cell could elicit iso‐orientation inhibition with a high impact between 100–400 and 800–1200 μm strong cross‐orientation inhibition at ∼400–800 μm, and oblique‐orientation inhibition between 300–500 and 700–900 μm from the parent soma. The non‐isotropic topography of large basket axons suggests a complex function for this cell class, possibly including inhibition related to orientation and direction selectivity depending on the location of the target cells and possible target selectivity.

Theta-Burst Transcranial Magnetic Stimulation Alters Cortical Inhibition

Human cortical excitability can be modified by repetitive transcranial magnetic stimulation (rTMS), but the cellular mechanisms are largely unknown. Here, we show that the pattern of delivery of theta-burst stimulation (TBS) (continuous versus intermittent) differently modifies electric activity and protein expression in the rat neocortex. Intermittent TBS (iTBS), but not continuous TBS (cTBS), enhanced spontaneous neuronal firing and EEG gamma band power. Sensory evoked cortical inhibition increased only after iTBS, although both TBS protocols increased the first sensory response arising from the resting cortical state. Changes in the cortical expression of the calcium-binding proteins parvalbumin (PV) and calbindin D-28k (CB) indicate that changes in spontaneous and evoked cortical activity following rTMS are in part related to altered activity of inhibitory systems. By reducing PV expression in the fast-spiking interneurons, iTBS primarily affected the inhibitory control of pyramidal cell output activity, while cTBS, by reducing CB expression, more likely affected the dendritic integration of synaptic inputs controlled by other classes of inhibitory interneurons. Calretinin, the third major calcium-binding protein expressed by another class of interneurons was not affected at all. We conclude that different patterns of TBS modulate the activity of inhibitory cell classes differently, probably depending on the synaptic connectivity and the preferred discharge pattern of these inhibitory neurons.