Cynthia D. Rittenhouse

The representation of brightness in primary visual cortex

Although neurons in primary visual cortex are sensitive to the spatial distribution and intensity of light, their responses have not been thought to correlate with the perception of brightness. Indeed, primary visual cortex is often described as an initial processing stage that sends information to higher cortical areas where perception of brightness, color, and form occurs. However, a significant percentage of neurons in primary visual cortex were shown to respond in a manner correlated with perceived brightness, rather than responding strictly to the light level in the receptive fields of the cells. This finding suggests that even at the first stage of visual cortical processing, spatial integration of information yields perceptual qualities that are only indirectly related to the pattern of illumination of the retina.

Sleep-Induced Changes in Associative Memory

The notion that dreaming might alter the strength of associative links in memory was first proposed almost 200 years ago. But no strong evidence of such altered associative links has been obtained. Semantic priming can be used to quantify the strength of associative links between pairs of words; it is thought to measure the automatic spread of activation from a node representing one word to nodes representing semantically related words. Semantic priming could thus be used to test for global alterations in the strengths of associative links across the wake-sleep cycle. Awakenings from REM and nonREM (NREM) sleep produce a period of state carry-over during which performance is altered as a result of the brains slow transition to full wakefulness, and cognitive testing in this period can provide information about the functioning of the brain during the prior sleep period. When subjects were tested across the nightbefore and after a nights sleep as well as immediately following forced awakenings from REM and NREM sleepweak priming (e.g., thief-wrong) was found to be state dependent (p = 0.016), whereas strong priming (e.g., hot-cold) was not (p = 0.89). Weak primes were most effective in the presleep and REM sleep conditions and least effective in NREM and postsleep conditions. Most striking are analyses comparing weak and strong priming within each wake-sleep state. Contrary to the normal pattern of priming, subjects awakened from REM sleep showed greater priming by weak primes than by strong primes (p = 0.01). This result was seen in each of three protocols. In contrast, strong priming exceeded weak priming in NREM sleep. The shift in weak priming seen after REM sleep awakenings suggests that cognition during REM sleep is qualitatively different from that of waking and NREM sleep and may reflect a shift in associative memory systems, a shift that we hypothesize underlies the bizarre and hyperassociative character of REM-sleep dreaming. Known changes in brainstem activity that control the transition into and maintenance of REM sleep provide a possible explanation of this shift.

Monocular deprivation induces homosynaptic long-term depression in visual cortex

Brief monocular deprivation during early postnatal development can lead to a depression of synaptic transmission that renders visual cortical neurons unresponsive to subsequent visual stimulation through the deprived eye. The Bienenstock–Cooper–Munro (BCM) theory proposes that homosynaptic mechanisms of long-term depression (LTD) account for the deprivation effects,. Homosynaptic depression, by definition, occurs only at active synapses. Thus, in contrast to the commonly held view that the synaptic depression caused by monocular deprivation is simply a result of retinal inactivity, this theoretical framework indicates that the synaptic depression may actually be driven by the residual activity in the visually deprived retina. Here we examine the validity of this idea by comparing the consequences of brief monocular deprivation by lid suture with those of monocular inactivation by intra-ocular treatment with tetrodotoxin. Lid suture leaves the retina spontaneously active, whereas tetrodotoxin eliminates all activity. In agreement with the BCM theory, our results show that monocular lid suture causes a significantly greater depression of deprived-eye responses in kitten visual cortex than does treatment with tetrodotoxin. These findings have important implications for mechanisms of experience-dependent plasticity in the neocortex.

Molecular basis for induction of ocular dominance plasticity

The most dramatic example of experience-dependent cortical plasticity is the shift in ocular dominance that occurs in visual cortex as a consequence of monocular deprivation during early postnatal life. Many of the basic properties of this type of synaptic plasticity have been described in detail. The important challenge that remains is to understand the molecular basis for these properties. By combining theoretical analysis with experiments in vivo and in vitro, some of the elementary molecular mechanisms for visual cortical plasticity have now been uncovered.

The representation of brightness in primary visual cortex.

Although neurons in primary visual cortex are sensitive to the spatial distribution and intensity of light, their responses have not been thought to correlate with the perception of brightness. Indeed, primary visual cortex is often described as an initial processing stage that sends information to higher cortical areas where perception of brightness, color, and form occurs. However, a significant percentage of neurons in primary visual cortex were shown to respond in a manner correlated with perceived brightness, rather than responding strictly to the light level in the receptive fields of the cells. This finding suggests that even at the first stage of visual cortical processing, spatial integration of information yields perceptual qualities that are only indirectly related to the pattern of illumination of the retina.

Self-Representation and Bizarreness in Children′s Dream Reports Collected in the Home Setting

Abstract We have conducted a home-based study of children′s dream reports in which parents used open-ended interviewing styles to collect 88 dream reports from their 4- to 10-year-old children in the comfortable and supportive environment of their own homes. Particular attention was paid to formal properties including characters (e.g., family members, human strangers, or frightening characters), settings, self-representation, and bizarreness. In contrast to previous studies, our data indicate that young children are able to give long, detailed reports of their dreams that share many formal characteristics with adult dream reports. Because this wide range of dream mentation is only revealed to trusted confidants in a familiar and comfortable environment, an important implication is that the sleep laboratory may not be the best source of naturalistic dream data.

A New Approach to Dream Bizarreness: Graphing Continuity and Discontinuity of Visual Attention in Narrative Reports

Abstract In this paper, a new method of quantitatively assessing continuity and discontinuity of visual attention is developed. The method is based on representing narrative information using graph theory. It is applicable to any type of narrative report. Since dream reports are often described as bizarre, and since bizarreness is partially characterized by discontinuities in plot, we chose to test our method on a set of dream data. Using specific criteria for identifying and arranging objects of visual attention, dream narratives from 10 subjects were obtained and mapped onto graphs. The interrater reliabilities were 76% (3/3 independent judges) and 91% (2/3 independent judges). Discontinuities in visual attention were quantified by plotting transitions from one part of a graph to another, which provided a spatiotemporal map of attention shifts within a narrative. This procedure was compared with other approaches to discontinuity and also applied to a set of 10 fantasy reports from the same subjects. The results showed that our method includes but transcends other approaches and has the capability to distinguish dream and fantasy reports. To our knowledge, the method provides the most rigorous and reliable measure to date of continuity and discontinuity of attention in narrative reports.

Response Sensitivity of Barrel Neuron Subpopulations to Simulated Thalamic Input

Our goal is to examine the relationship between neuron- and network-level processing in the context of a well-studied cortical function, the processing of thalamic input by whisker-barrel circuits in rodent neocortex. Here we focus on neuron-level processing and investigate the responses of excitatory and inhibitory barrel neurons to simulated thalamic inputs applied using the dynamic clamp method in brain slices. Simulated inputs are modeled after real thalamic inputs recorded in vivo in response to brief whisker deflections. Our results suggest that inhibitory neurons require more input to reach firing threshold, but then fire earlier, with less variability, and respond to a broader range of inputs than do excitatory neurons. Differences in the responses of barrel neuron subtypes depend on their intrinsic membrane properties. Neurons with a low input resistance require more input to reach threshold but then fire earlier than neurons with a higher input resistance, regardless of the neurons classification. Our results also suggest that the response properties of excitatory versus inhibitory barrel neurons are consistent with the response sensitivities of the ensemble barrel network. The short response latency of inhibitory neurons may serve to suppress ensemble barrel responses to asynchronous thalamic input. Correspondingly, whereas neurons acting as part of the barrel circuit in vivo are highly selective for temporally correlated thalamic input, excitatory barrel neurons acting alone in vitro are less so. These data suggest that network-level processing of thalamic input in barrel cortex depends on neuron-level processing of the same input by excitatory and inhibitory barrel neurons.

Emotion and Visual Imagery in Dream Reports: A Narrative Graphing Approach

Abstract To test the notion that shifts in visual imagery and attention are correlated with experiences of emotion, we studied 10 dream reports using an affirmative probe of emotion and a quantitative measure of plot discontinuity. We found that emotion, especially changes in emotion, are correlated with discontinuities in visual imagery. These correlations are quantified using a new graph theoretical method for analyzing narrative reports.

Survival skills and ethics training for graduate students: A graduate student perspective

Graduate students in the sciences must develop practical skills geared toward scientific survival and success. This is particularly true now, given the paucity of research funds and jobs. Along with more elementary skills, research ethics should be an integral part of students’ scientific training. Survival skills include research skills, communication skills, general efficiency, and preparation for post-graduate work. Ethics training covers guidelines for use of animal and human subjects, data treatment, disclosure, credit issues, conflicts of interest, and response to misconduct. The objective of this paper is to describe, from a graduate student’s perspective, the need for survival and ethics training in graduate programs and to raise both faculty and student awareness of the possibilities for explicit instruction of these skills. Many survival skills and ethical practices will be learned without explicit direction and some are already part of standard training; but, this is not the case for all students or for all skills, so specific instruction is a necessity. Research faculty can use their own experience to help students to develop the proficiencies they will need to succeed.