Stimulus Discrimination Performance Research Articles

Cathodal tDCS stimulation of left anterior temporal lobe eliminates cross-category color discrimination response time advantage

The linguistic category-advantage in color perception refers to better discrimination performance for stimuli that belong to different categories (e.g., green vs. blue) than equidistant stimuli from the same category (e.g., blue). Despite the robust nature of category-advantage in color perception, the related cognitive and neural mechanisms are not fully understood. Some views attribute this effect to early alteration of visual processing of color while others attribute it to post-perceptual conceptual processing. The current study investigated the causal role of the left anterior temporal lobe (ATL), as a post-perceptual semantic hub, in categorical color perception. We modulated the activity of the left ATL via cathodal tDCS or sham stimulation (within-subject) while participants were discriminating between successive presentations of color patches. Without stimulation, we found a category-advantage effect in both accuracy and response times. The inhibition of left ATL eliminated the category-advantage effect in terms of RTs but not accuracies. Our results point at the causal role of ATL in categorical color perception and provide indirect support for a post-perceptual processing account of this robust phenomenon.

The role of mechanosensory T cells for stimulus encoding in the local bend network of the leech

Accurate behavioral responses to sensory stimuli require reliable encoding and processing of stimulus properties in the underlying neuronal network. The relatively simple nervous system of the leech is able to react with surprising precision when the skin is touched: With their local bend response [1], leeches discriminate behaviorally between touch location differences of 9° [2], corresponding to a distance of less than 1 mm. The underlying network consists of one layer of sensory neurons, approximately 20 interneurons and a layer of motor neurons [1]. Studies investigating the local bend mainly focused on one of the three mechanosensory cell types, the pressure (P) cells [1,2]. But decoding experiments revealed a discrepancy between P cell activity and the behavioral performance [2]. Therefore, we investigated how the mechanosensory touch (T) cells, respond to touch stimulus properties and influence responses of local bend interneurons. Using a semi-intact preparation, we elicited the local bend response with mechanical stimulation of the skin. Simultaneously, we performed intracellular double recordings from T cells and interneurons 157 or 159, which are involved in the local bend network [3]. We found that T cells respond with characteristic bursts to the onset of touch stimulation. The first T cell spikes are generated with an extremely high temporal precision and short response latency for a broad range of touch stimulus intensities. Local bend interneurons 157 and 159 get input from T cells and produce characteristic EPSPs with short response latencies in response to tactile stimulation. In particular EPSPs of cell 159 follow T cell bursts. For quantitative data analysis, we used two maximum likelihood approaches of stimulus estimation: The classification approach assigned each response trial to the most probable of all possible stimuli. It provides a measure, how well a certain response feature (e.g. spike count, latency, EPSP amplitude) encodes a specific stimulus property (location, intensity, duration). The pairwise discrimination approach compares responses to two stimuli and quantifies minimal stimulus differences, which could be detected based on the neuronal response features. These methods revealed that the relative latency of two T cells leads to the best estimation of stimulus locations (for tested intensities up to 50 mN). For interneurons, graded response features (e.g. EPSP integral and amplitude) allow good stimulus discrimination performance, even for small distances of touch locations. Additionally, we started building a computational model of the local bend network. We tried to fit the parameters of the Izhikevich model [4] to intracellularly recorded T cell responses. However, the depolarized resting potential (compared to cortical neurons) of this invertebrate cell required modification of the model. Moreover, we were not able to find a parameter range reproducing the characteristic T-cell bursts in response to stimulus onset. None of the tested parameter sets yielded stimulus-dependent response latencies, which are essential for stimulus encoding. Hence, the Izhikevich model is not well suited for modeling the response properties of an invertebrate sensor cell, which are relevant for the encoding of sensory stimuli.

Using spike train distances to identify the most discriminative neuronal subpopulation

During the last decade spike train distances [1-4] have become an essential means to characterize neural coding in a wide range of neurophysiological contexts. In a typical setup different stimuli are presented repeatedly and spike train distances are used to carry out a pairwise similarity analysis in order to evaluate whether responses to the same stimulus exhibit smaller distances than responses to different stimuli. With the increasing availability of multi-neuron recordings these kinds of analyses can now be performed not just for individual neurons but rather for simultaneously recorded neuronal populations. Recently, the bivariate Victor-Purpura and the van Rossum distances [1,2] have been extended to quantify dissimilarities between multi-unit responses. These new population measures [5,6] have been designed such that they can estimate the discrimination performance of either the population as a whole (summed population”) or of individual neurons (labelled line”) or of interpolations between these two extremes. However, the question that these approaches fail to answer is the following: In cases the two extremes fail, which subpopulation within the larger population discriminates the presented stimuli best? In this study we thus follow a complimentary approach and present an algorithm which addresses exactly this question. The brute-force approach of calculating the pairwise distance matrices and the stimulus discrimination performance for every possible neuronal subpopulation is not feasible even for moderate numbers of neurons. Instead, our algorithm relies on an iterative scheme which considerably restricts the number of subpopulations for which the pairwise distance matrices and the stimulus discrimination performance actually have to be calculated. The spike train distance that we use to evaluate whether the responses elicited by different stimuli can be distinguished is the SPIKE-distance [4,7]. Whereas distances like the Victor-Purpura or the van Rossum spike train distance rely on a time-scale parameter, the SPIKE-distance is parameter-free and time-scale independent. This allows for easy comparability of results obtained for vastly different firing rates (which depend on the size of the pooled subpopulation). In contrast to the ISI-distance [3] it is sensitive to spike timing which is an important property that will be needed in some of the setups used.

Theta-burst transcranial magnetic stimulation to the prefrontal cortex impairs metacognitive visual awareness

We used a recently developed protocol of transcranial magnetic stimulation (TMS), theta-burst stimulation, to bilaterally depress activity in the dorsolateral prefrontal cortex as subjects performed a visual discrimination task. We found that TMS impaired subjects' ability to discriminate between correct and incorrect stimulus judgments. Specifically, after TMS subjects reported lower visibility levels for correctly identified stimuli, as if they were less fully aware of the quality of their visual information processing. A signal detection theory analysis confirmed that the results reflect a change in metacognitive sensitivity, not just response bias. The effect was specific to metacognition; TMS did not change stimulus discrimination performance, ruling out alternative explanations such as TMS impairing visual attention. Together these results suggest that activations in the prefrontal cortex in brain imaging experiments on visual awareness are not epiphenomena, but rather may reflect a critical metacognitive process.

GABA Concentration Is Reduced in Visual Cortex in Schizophrenia and Correlates with Orientation-Specific Surround Suppression

The neural mechanisms underlying cognitive deficits in schizophrenia remain essentially unknown. The GABA hypothesis proposes that reduced neuronal GABA concentration and neurotransmission results in cognitive impairments in schizophrenia. However, few in vivo studies have directly examined this hypothesis. We used magnetic resonance spectroscopy (MRS) at high field to measure visual cortical GABA levels in 13 subjects with schizophrenia and 13 demographically matched healthy control subjects. We found that the schizophrenia group had an approximately 10% reduction in GABA concentration. We further tested the GABA hypothesis by examining the relationship between visual cortical GABA levels and orientation-specific surround suppression (OSSS), a behavioral measure of visual inhibition thought to be dependent on GABAergic synaptic transmission. Previous work has shown that subjects with schizophrenia exhibit reduced OSSS of contrast discrimination (Yoon et al., 2009). For subjects with both MRS and OSSS data (n = 16), we found a highly significant positive correlation (r = 0.76) between these variables. GABA concentration was not correlated with overall contrast discrimination performance for stimuli without a surround (r = -0.10). These results suggest that a neocortical GABA deficit in subjects with schizophrenia leads to impaired cortical inhibition and that GABAergic synaptic transmission in visual cortex plays a critical role in OSSS.

Right or wrong, familiar or novel in pictorial list discrimination learning.

The interaction between nonassociative learning (presentation frequencies) and associative learning (reinforcement rates) in stimulus discrimination performance was investigated. Subjects were taught to discriminate lists of visual pattern pairs. When they chose the stimulus designated as right they were symbolically rewarded and when they chose the stimulus designated as wrong they were symbolically penalised. Subjects first learned one list and then another list. For a "right" group the pairs of the second list consisted of right stimuli from the first list and of novel wrong stimuli. For a "wrong" group it was the other way round. The right group transferred some discriminatory performance from the first to the second list while the control and wrong groups initially only performed near chance with the second list. When the first list involved wrong stimuli presented twice as frequently as right stimuli, the wrong group exhibited a better transfer than the right group. In a final experiment subjects learned lists which consisted of frequent right stimuli paired with scarce wrong stimuli and frequent wrong stimuli paired with scarce right stimuli. In later test trials these stimuli were shown in new combinations and additionally combined with novel stimuli. Subjects preferred to choose the most rewarded stimuli and to avoid the most penalised stimuli when the test pairs included at least one frequent stimulus. With scarce/scarce or scarce/novel stimulus combinations they performed less well or even chose randomly. A simple mathematical model that ascribes stimulus choices to a Cartesian combination of stimulus frequency and stimulus value succeeds in matching all these results with satisfactory precision.

Visual discrimination learning in mentally handicapped adults: comparative effects of two-choice and multiple-choice training methods on stimulus generalization performance.

The hypothesis that in mentally handicapped subjects a multi-choice discrimination learning method would result in superior generalization performance to the traditional two-choice method was investigated experimentally. Twenty-four mentally handicapped adults matched for age, intelligence and duration of institutionalization were divided into two equal groups and given visual discrimination training using a differential reinforcement, prompt-fading procedure. The groups were allocated separately to a two-choice and four-choice training condition in which the discriminative stimulus was associated with one and three non-discriminative stimuli respectively. Following training, stimulus discrimination performance was compared on a generalization test comprising increasing levels of distraction. The results showed a clear advantage for multi-choice training over two-choice. However, it was found that during generalization testing discriminative performance deteriorated as a function of increasing distraction in both groups, suggesting an underlying selective attention defect. Such a defect may be characteristic of stimulus generalization performance in the mentally retarded. Multi-choice discrimination learning appeared partly to counteract major generalization failure when the ratio of non-discriminative to discriminative stimuli was high.