Andrea Antal

Schizophrenics know more than they can tell: probabilistic classification learning in schizophrenia.

BACKGROUNDnPrevious studies have demonstrated impaired explicit and preserved implicit memory functions in schizophrenia. However, it is less clear whether schizophrenics can learn complex information (e.g. probabilistic stimulus response associations) with or without access for conscious recollection. In this study we applied a classification learning task to assess explicit and implicit processes concurrently.nnnMETHODSnTwo test procedures were administered to 40 schizophrenic subjects and 20 healthy volunteers: a probabilistic classification learning (PCL) task to evaluate implicit memory functions; and a category cue recognition test to investigate the explicit memory system. The PCL task included feedback guided category learning of geometrical shapes. These shapes were called category cues, predicting class membership with certain probabilities. The gradual increase of categorization performance during the feedback learning was a potentially implicit process, whereas the subsequent recognition of category cues required explicit memory functions.nnnRESULTSnThe schizophrenic patients improved their categorization performance to a similar extent to the controls, but they failed to recognize the category cues. Memory performances were independent of the positive and negative symptoms.nnnCONCLUSIONSnPatients with schizophrenia were able to establish representations of complex categories, but these remained unconscious. This is consistent with earlier reports, suggesting damaged explicit and spared implicit memory in schizophrenia.

Early and late components of visual categorization: an event-related potential study

We examined the characteristics of early and late components of event-related potentials (ERPs) accompanying the visual categorization of natural scenes. In the first experiment, ERPs were recorded in an animal-non-animal categorization task (CT), whereas the second experiment included a spatial frequency discrimination task (DT). In the CT, there were more negative potentials for non-animals in the time windows of 150-250 ms (N1) and 350-500 ms (N2), and a more positive potential for animals in the time window of 250-350 ms (P2). In the DT, spatial frequency gratings evoked only a short-duration difference N1 at the frontal sites. Our results suggest that N1 may be related to higher-level categorization processes.

Visual information processing in patients with schizophrenia: evidence for the impairment of central mechanisms

Patients with schizophrenia are especially impaired in the detection of spatial location if the briefly presented target stimulus is followed by a mask in a close temporal proximity (target location backward masking (BM) paradigm). It has been suggested that this phenomenon is related to the impairment of low spatial and high temporal frequency-sensitive transient (magnocellular) visual channels. To test this hypothesis, we measured target location BM and visual contrast sensitivity (CS) in clinically remitted patients with schizophrenia. In the BM task, subjects were asked to indicate the position of letters appearing at four possible spatial locations. In the CS test, a two-alternative forced choice method was used to measure the minimal contrast level required for the detection of horizontal gratings set at low spatial and high temporal frequencies (0.5 cycle/degree and 8 Hz, respectively). We found that the schizophrenia patients with normal CSs (spared transient channel functions) showed a marked deficit in the target location BM task. This suggests that the abnormality of subcortical transient channels does not explain some visual information processing dysfunctions in schizophrenia. Instead, deficient cortical interactions of rapidly changing environmental signals may be involved.

Dopamine D2 receptor blockade alters the primary and cognitive components of visual evoked potentials in the monkey, Macaca fascicularis

The effects of sulpiride, a dopamine D2 receptor antagonist, were studied on visual event-related potentials in a monkey performing in a visual oddball task in order to investigate receptor specific mechanisms in visuo-cognitive processes. Following the injection of 0.35 mg/kg sulpiride i.m., the amplitude and latency of the primary (P100) and cognitive (P300) components did not change significantly. When 1.05 mg/kg sulpiride was administered, the latency of the primary and cognitive components increased. The amplitude of the P100 component decreased, while that of the P300 component increased. These data suggest that D2 receptors play an important role in visuo-cognitive processes in both physiological and pathological conditions such as Parkinsons disease and schizophrenia.

Early visual impairment is independent of the visuocognitive and memory disturbances in Alzheimer's disease.

Static and dynamic contrast sensitivity (SCS and DCS), semantic object identification, and verbal recall functions were evaluated in 20 Alzheimers disease (AD) patients and in 20 control subjects. We found general SCS and DCS loss in the 0.48-14.34 c deg-1 spatial frequency range. In relation to the cognitive functions, semantic object identification was intact, whereas explicit memory was markedly impaired in the AD group. There was no significant correlation between the CS and the memory disturbances. The results suggest that early visual impairment and higher-level cognitive disturbances are independent in AD.

Electrophysiological correlates of visual categorization: evidence for cognitive dysfunctions in early Parkinson's disease.

Our aim was to examine the electrophysiological correlates of perceptual categorization in Parkinsons disease (PD). We recorded visual event-related potentials (ERPs) in a natural scene categorization task in drug naive idiopathic PD patients and healthy control subjects. In the control group, there was a significant early difference (150-250 ms poststimulus) between ERPs elicited by pictures containing animals and scenes without animals. In spite of relatively preserved basic-level visual functions, this was not observable in the PD group. These results raise the possibility for striatal contributions to visual categorization and may provide a novel protocol for further clinical studies.

Sodium bromide treatment influences the plasticity of somatosensory responses in the rat cortex as induced by enucleation

Effects of sodium bromide were studied on central neuroplasticity induced by early binocular enucleation. It has previously been found that enucleation on the day of birth, but not later than the first postnatal week, resulted in changes in the occipital cortex, such as the invasion of somatosensory evoked activity into the visual cortex areas. The present results showed that sodium bromide treatment extended at least up to 15 days after birth, the critical period during which somatosensory projections could be modified by visual deafferentation. Together with observations of Frost [J. comp. Neurol. (1981) 203, 227-256; Devl Brain Res. (1982) 3, 627-636], the present results suggest a mutual dependency of visual and somatosensory projection development. The present study is the first demonstration that the critical period of development, during which a specific type of neural plasticity can be induced, may be prolonged by pharmacological means, i.e. by chronic treatment with sodium bromide.

Event-related potentials from a visual categorization task.

Investigation of the neural bases of human perceptual categorization is a continuously growing territory of visual sciences. Recently, several papers appeared on the cortical bases of human categorization performance, using functional brain imaging techniques such as positron emission tomography (PET) or functional magnetic resonance imaging (fMRI). However, the cheaper commercially available laboratory tests are not always the best methods to study multiple aspects of visual categorization. In this study, we describe the electrophysiological correlates of natural scene categorization in humans. The subjects task was to decide whether briefly presented natural scenes contained animal or non-animal items. Analysis of event-related potentials (ERPs) recorded during the categorization task, revealed more negative potentials for non-animal stimuli (NAS) in the time windows of 150-250 ms (N1) and 350-500 ms (N2), and more positive potentials for animal stimuli (AS) in the time window of 250-350 ms (P2). Our work provides an inexpensive noninvasive method to study both early perceptual and late phases of semantic information processing by recording ERPs during the categorization of natural scenes.

Visual discrimination and P300 are affected in parallel by cholinergic agents in the behaving monkey.

We studied the acute effect of cholinergic agents, scopolamine alone and scopolamine pretreatment and acetyl-L-carnitine (ALC) on visual event-related potentials (ERPs) in a monkey performing in a go/no-go visual oddball discrimination task. The same monkey was studied repeatedly for a period extending over 1.5 years. Scopolamine is known to cause cognitive impairment in primates. In the concentrations used, primary evoked potentials did not change significantly whereas P300 latency increased maximally 20-40 min following scopolamine administration. Acetyl-L-carnitine on its own increased P300 amplitude and decreased its latency. When ALC administration was preceded by scopolamine, both P300 latency and amplitude maximally increased in 25-45 min. Apparently ALCs effect on the latency and amplitude of visual P300 of the monkey is largely via muscarinic mechanisms. Our results suggest that scopolamine may provide a valuable tool to separate cognitive from primary visual processes.

Light therapy increases visual contrast sensitivity in seasonal affective disorder

The purpose of this study was to investigate the effects of light therapy on visual contrast sensitivity in patients with seasonal affective disorder (n=10) and healthy control subjects (n=10). Static and dynamic visual contrast sensitivity was measured using a Venus system before and after 4 weeks of light therapy (10,000 lux, 30 min, 5 times a week). Light therapy increased static visual contrast sensitivity in the patients. We found no significant difference between the patients and controls either before or after light therapy. These results raise the possibility that light therapy induces retinal sensitization in seasonal affective disorder.