Neural signature of the perceptual decision in the neural population responses of the inferior temporal cortex.

Rapid categorization of visual objects is critical for comprehending our complex visual world. The role of individual cortical neurons and neural populations in categorizing visual objects during passive vision has previously been studied. However, it is unclear whether and how perceptually guided behaviors affect the encoding of stimulus categories by neural population activity in the higher visual cortex. Here we studied the activity of the inferior temporal (IT) cortical neurons in macaque monkeys during both passive viewing and categorization of ambiguous body and object images. We found enhanced category information in the IT neural population activity during the correct, but not wrong, trials of the categorization task compared to the passive task. This encoding enhancement was task difficulty dependent with progressively larger values in trials with more ambiguous stimuli. Enhancement of IT neural population information for behaviorally relevant stimulus features suggests IT neural networks' involvement in perceptual decision-making behavior.

The effect of age and smoking on the hippocampus and memory in late middle age.

From middle age the hippocampus atrophies at an accelerating rate. Factors that further this acceleration may hasten memory decline and the onset of memory disorder. We studied associations between smoking history, age, ApoE e4 genotype, vascular risk factors, hippocampal volume, and cognition in 67 middle-aged subjects (mean age = 56 years) who were offspring of parents with dementia. Subjects underwent isotropic T1-weighted 3 T MRI brain scanning with FreeSurfer volumetric data extraction for the hippocampus, a neuropsychological assessment battery, extensive medical data collection, and ApoE genotyping. ApoE e4, vascular risk variables, and alcohol history were unrelated to hippocampal volume. Hippocampal volume correlated negatively with age and positively with memory performance, but not with global cognition. Aging diminished hippocampal volume by 0.52% per year. Female subjects (only two males smoked) with a heavy smoking history (≥ 9.5 pack-years; n = 11) exhibited hippocampal volumes that were 7.4% smaller than the volumes of females (n = 37) with a light or no smoking history. In our sample by late middle age, a history of moderate to heavy smoking is associated with hippocampal atrophy equivalent to 12 years of aging. Since only a small number of subjects within the sample have a smoking history, validation of this finding in larger samples is desirable.

Hyperinsulinemia and elevated systolic blood pressure independently predict white matter hyperintensities with associated cognitive decrement in the middle-aged offspring of dementia patients.

Cerebrovascular disease is an independent risk factor for dementia that may also be synergistic with Alzheimer's disease. In recent years attention has switched from cerebral infarcts to microvascular disease as the primary cause of cerebrovascular cognitive decline, with damage to the white matter the primary mechanism. Uncertainties remain regarding the risks posed by different types vascular threat, the extent to which cerebrovascular damage occurs in middle age, and whether relatively "normal" amounts of white matter damage are accompanied by meaningful degrees of cognitive decline. We explored these issues via laboratory, cardiovascular, cognitive, and magnetic resonance imaging (MRI) data in 67 middle-aged cognitively normal offspring of dementia patients. The sample was enriched for vascular risk. Plasma insulin, 24-h systolic blood pressure, body mass index, age, and % small dense LDL cholesterol were the strongest correlates of MRI white matter hyperintensity (WMH) volume. With shared variance controlled for, 24 h systolic BP, plasma insulin, and age remained as significant predictors of WMH volume. An interaction variable (24 h BP * insulin) did not improve the prediction of WMH. WMH volume correlated negatively with cognition. No evidence for an ApoE ε4 effect emerged for either WMH or cognition. Hypertension and hyperinsulinemia appear to pose independent, consequential threats to the cerebral small vessel vasculature in middle age, reflected in the presence of areas of WMH on MRI scans. Our data show that even modest WMH volumes in middle age are associated with cognitive decrement, underscoring the importance of aggressive treatment and lifestyle modifications to address vascular risk throughout adulthood.

Sleep abnormalities associated with alcohol, cannabis, cocaine, and opiate use: a comprehensive review.

Sleep abnormalities are associated with acute and chronic use of addictive substances. Although sleep complaints associated with use and abstinence from addictive substances are widely recognized, familiarity with the underlying sleep abnormalities is often lacking, despite evidence that these sleep abnormalities may be recalcitrant and impede good outcomes. Substantial research has now characterized the abnormalities associated with acute and chronic use of alcohol, cannabis, cocaine, and opiates. This review summarizes this research and discusses the clinical implications of sleep abnormalities in the treatment of substance use disorders.

High baseline activity in inferior temporal cortex improves neural and behavioral discriminability during visual categorization.

Spontaneous firing is a ubiquitous property of neural activity in the brain. Recent literature suggests that this baseline activity plays a key role in perception. However, it is not known how the baseline activity contributes to neural coding and behavior. Here, by recording from the single neurons in the inferior temporal cortex of monkeys performing a visual categorization task, we thoroughly explored the relationship between baseline activity, the evoked response, and behavior. Specifically we found that a low-frequency (<8 Hz) oscillation in the spike train, prior and phase-locked to the stimulus onset, was correlated with increased gamma power and neuronal baseline activity. This enhancement of the baseline activity was then followed by an increase in the neural selectivity and the response reliability and eventually a higher behavioral performance.

Behavioral demand modulates object category representation in the inferior temporal cortex.

Visual object categorization is a critical task in our daily life. Many studies have explored category representation in the inferior temporal (IT) cortex at the level of single neurons and population. However, it is not clear how behavioral demands modulate this category representation. Here, we recorded from the IT single neurons in monkeys performing two different tasks with identical visual stimuli: passive fixation and body/object categorization. We found that category selectivity of the IT neurons was improved in the categorization compared with the passive task where reward was not contingent on image category. The category improvement was the result of larger rate enhancement for the preferred category and smaller response variability for both preferred and nonpreferred categories. These specific modulations in the responses of IT category neurons enhanced signal-to-noise ratio of the neural responses to discriminate better between the preferred and nonpreferred categories. Our results provide new insight into the adaptable category representation in the IT cortex, which depends on behavioral demands.

Neural representation of ambiguous visual objects in the inferior temporal cortex.

Inferior temporal (IT) cortex as the final stage of the ventral visual pathway is involved in visual object recognition. In our everyday life we need to recognize visual objects that are degraded by noise. Psychophysical studies have shown that the accuracy and speed of the object recognition decreases as the amount of visual noise increases. However, the neural representation of ambiguous visual objects and the underlying neural mechanisms of such changes in the behavior are not known. Here, by recording the neuronal spiking activity of macaque monkeys' IT we explored the relationship between stimulus ambiguity and the IT neural activity. We found smaller amplitude, later onset, earlier offset and shorter duration of the response as visual ambiguity increased. All of these modulations were gradual and correlated with the level of stimulus ambiguity. We found that while category selectivity of IT neurons decreased with noise, it was preserved for a large extent of visual ambiguity. This noise tolerance for category selectivity in IT was lost at 60% noise level. Interestingly, while the response of the IT neurons to visual stimuli at 60% noise level was significantly larger than their baseline activity and full (100%) noise, it was not category selective anymore. The latter finding shows a neural representation that signals the presence of visual stimulus without signaling what it is. In general these findings, in the context of a drift diffusion model, explain the neural mechanisms of perceptual accuracy and speed changes in the process of recognizing ambiguous objects.

Neuronal spike train analysis in likelihood space.

BACKGROUND: Conventional methods for spike train analysis are predominantly based on the rate function. Additionally, many experiments have utilized a temporal coding mechanism. Several techniques have been used for analyzing these two sources of information separately, but using both sources in a single framework remains a challenging problem. Here, an innovative technique is proposed for spike train analysis that considers both rate and temporal information. METHODOLOGY/PRINCIPAL FINDINGS: Point process modeling approach is used to estimate the stimulus conditional distribution, based on observation of repeated trials. The extended Kalman filter is applied for estimation of the parameters in a parametric model. The marked point process strategy is used in order to extend this model from a single neuron to an entire neuronal population. Each spike train is transformed into a binary vector and then projected from the observation space onto the likelihood space. This projection generates a newly structured space that integrates temporal and rate information, thus improving performance of distribution-based classifiers. In this space, the stimulus-specific information is used as a distance metric between two stimuli. To illustrate the advantages of the proposed technique, spiking activity of inferior temporal cortex neurons in the macaque monkey are analyzed in both the observation and likelihood spaces. Based on goodness-of-fit, performance of the estimation method is demonstrated and the results are subsequently compared with the firing rate-based framework. CONCLUSIONS/SIGNIFICANCE: From both rate and temporal information integration and improvement in the neural discrimination of stimuli, it may be concluded that the likelihood space generates a more accurate representation of stimulus space. Further, an understanding of the neuronal mechanism devoted to visual object categorization may be addressed in this framework as well.