Biomedical subjects

Michael J Berry

Publications and source records attributed to Michael J Berry.

14 recordsLinked to original sources

PubMed2006

Selectivity for multiple stimulus features in retinal ganglion cells.

Under normal viewing conditions, retinal ganglion cells transmit to the brain an encoded version of the visual world. The retina parcels the visual scene into an array of spatiotemporal features, and each ganglion cell conveys information about a small set of these features. We study the temporal features represented by salamander retinal ganglion cells by stimulating with dynamic spatially uniform flicker and recording responses using a multi-electrode array. While standard reverse correlation methods determine a single stimulus feature--the spike-triggered average--multiple features can be relevant to spike generation. We apply covariance analysis to determine the set of features to which each ganglion cell is sensitive. Using this approach, we found that salamander ganglion cells represent a rich vocabulary of different features of a temporally modulated visual stimulus. Individual ganglion cells were sensitive to at least two and sometimes as many as six features in the stimulus. While a fraction of the cells can be described by a filter-and-fire cascade model, many cells have feature selectivity that has not previously been reported. These reverse models were able to account for 80-100% of the information encoded by ganglion cells.

Algorithms↗

PubMed2006

How much the eye tells the brain.

In the classic "What the frog's eye tells the frog's brain," Lettvin and colleagues showed that different types of retinal ganglion cell send specific kinds of information. For example, one type responds best to a dark, convex form moving centripetally (a fly). Here we consider a complementary question: how much information does the retina send and how is it apportioned among different cell types? Recording from guinea pig retina on a multi-electrode array and presenting various types of motion in natural scenes, we measured information rates for seven types of ganglion cell. Mean rates varied across cell types (6-13 bits . s(-1)) more than across stimuli. Sluggish cells transmitted information at lower rates than brisk cells, but because of trade-offs between noise and temporal correlation, all types had the same coding efficiency. Calculating the proportions of each cell type from receptive field size and coverage factor, we conclude (assuming independence) that the approximately 10(5) ganglion cells transmit on the order of 875,000 bits . s(-1). Because sluggish cells are equally efficient but more numerous, they account for most of the information. With approximately 10(6) ganglion cells, the human retina would transmit data at roughly the rate of an Ethernet connection.

Action Potentials↗

PubMed2006

Weak pairwise correlations imply strongly correlated network states in a neural population.

Biological networks have so many possible states that exhaustive sampling is impossible. Successful analysis thus depends on simplifying hypotheses, but experiments on many systems hint that complicated, higher-order interactions among large groups of elements have an important role. Here we show, in the vertebrate retina, that weak correlations between pairs of neurons coexist with strongly collective behaviour in the responses of ten or more neurons. We find that this collective behaviour is described quantitatively by models that capture the observed pairwise correlations but assume no higher-order interactions. These maximum entropy models are equivalent to Ising models, and predict that larger networks are completely dominated by correlation effects. This suggests that the neural code has associative or error-correcting properties, and we provide preliminary evidence for such behaviour. As a first test for the generality of these ideas, we show that similar results are obtained from networks of cultured cortical neurons.

Action Potentials↗

PubMed2006

The Reconditioning Exercise and Chronic Obstructive Pulmonary Disease Trial II (REACT II): rationale and study design for a clinical trial of physical activity among individuals with chronic obstructive pulmonary disease.

Chronic obstructive pulmonary disease (COPD) is the fourth leading cause of mortality in the United States. In addition, persons with COPD are at risk for lower levels of physical activity, leading to further morbidity and mortality. Several studies have demonstrated that long-term exercise therapy confers benefits upon physical functioning among patients with COPD, and some studies indicate that embedding cognitive-behavioral interventions into group-mediated exercise programs is useful in promoting compliance to activity recommendations. However, compliance to long-term activity is low among COPD patients, and the effectiveness of behavioral interventions to enhance long-term activity among these patients has not been extensively explored. Thus, the primary objective of the Reconditioning Exercise and COPD Trial II (REACT II) trial is to determine whether a group-mediated cognitive-behavioral intervention will result in increased physical activity after 12 months, compared to a standard exercise therapy experience among older adults with COPD. The cognitive-behavioral intervention is designed to promote independent physical activity by encouraging participants to self-regulate physical activity with minimal dependence upon staff. The primary study outcome is kcal expended per week in moderate physical activity, and the study is designed to provide 90% power to detect a 400 kcal/week difference in moderate energy expenditure between the two intervention groups. Other outcomes to be compared between the two interventions include physical function, self-reported physical disability, health-related quality of life, exercise capacity, body composition and inflammatory mediators.

Body Composition↗

PubMed2006

Use of peak oxygen consumption in predicting physical function and quality of life in COPD patients.

OBJECTIVE: To determine whether peak oxygen consumption (Vo(2)peak) adds to the power of FEV(1) in predicting physical function and quality of life in COPD patients. DESIGN: Single-center cross-sectional study. METHODS: Subjects included 291 COPD patients who completed pulmonary function testing, a graded exercise test, a 6-min walk, and stair climb test to assess physical function; a questionnaire assessing self-reported physical function; and a disease-specific, health-related quality-of-life questionnaire. Hierarchical multiple regression analysis was used to determine the contribution of Vo(2)peak in predicting physical function and quality of life after accounting for FEV(1). RESULTS: After accounting for FEV(1), Vo(2)peak added significantly to the prediction of 6-min walk distance (R(2) increased by 0.395 [p < 0.005]); stair climb time (R(2) increased by 0.262 [p < 0.005]); self-reported function (R(2) increased by 0.109 [p < 0.005]); and health-related quality-of-life domain of mastery (R(2) increased by 0.044 [p < 0.005]). Only Vo(2)peak was found to significantly predict the health-related quality-of-life domain of fatigue (R(2) = 0.094 [p < 0.005]). CONCLUSION: After controlling for FEV(1), Vo(2)peak adds significantly to the prediction of physical function and health-related quality-of-life domain of mastery in COPD patients. These results provide additional support for the use of Vo(2)peak in the multidimensional assessment of COPD patients.

Activities of Daily Living↗

PubMed2005

Functional organization of ganglion cells in the salamander retina.

Recently, we reported a novel technique for recording all of the ganglion cells in a retinal patch and showed that their receptive fields cover visual space roughly 60 times over in the tiger salamander. Here, we carry this analysis further and divide the population of ganglion cells into functional classes using quantitative clustering algorithms that combine several response characteristics. Using only the receptive field to classify ganglion cells revealed six cell types, in agreement with anatomical studies. Adding other response measures served to blur the distinctions between these cell types rather than resolve further classes. Only the biphasic off type had receptive fields that tiled the retina. Even when we attempted to split these classes more finely, ganglion cells with almost identical functional properties were found to have strongly overlapping spatial receptive fields. A territorial spatial organization, where ganglion cell receptive fields tend to avoid those of other cells of the same type, was only found for the biphasic off cell. We further studied the functional segregation of the ganglion cell population by computing the amount of visual information shared between pairs of cells under natural movie stimulation. This analysis revealed an extensive mixing of visual information among cells of different functional type. Together, our results indicate that the salamander retina uses a population code in which every point in visual space is represented by multiple neurons with subtly different visual sensitivities.

Action Potentials↗

PubMed2005

Redundancy in the population code of the retina.

We have explored the manner in which the population of retinal ganglion cells collectively represent the visual world. Ganglion cells in the salamander were recorded simultaneously with a multielectrode array during stimulation with both artificial and natural visual stimuli, and the mutual information that single cells and pairs of cells conveyed about the stimulus was estimated. We found significant redundancy between cells spaced as far as 500 mum apart. When we used standard methods for defining functional types, only ON-type and OFF-type cells emerged as truly independent information channels. Although the average redundancy between nearby cell pairs was moderate, each ganglion cell shared information with many neighbors, so that visual information was represented approximately 10-fold within the ganglion cell population. This high degree of retinal redundancy suggests that design principles beyond coding efficiency may be important at the population level.

Animals↗

PubMed2004

Perceived difficulty, importance, and satisfaction with physical function in COPD patients.

BACKGROUND: Research suggests that patients' satisfaction with their physical functioning (SPF) is a critical component of HRQL. This study was designed to examine the extent to which perceptions of physical function and the value placed on physical function are related to satisfaction ratings. The sample consisted of older adults suffering from a progressively debilitating disease, chronic obstructive pulmonary disease (COPD). METHODS: During baseline assessments, COPD patients participating in a randomized controlled physical activity trial completed measures of SPF, perceived difficulty, and perceived importance. RESULTS: An ANCOVA controlling for age and gender indicated that perceived difficulty, perceived importance, and their interaction accounted for 43% of the variance in SPF. Additionally, participants were most satisfied with important tasks that they performed with little difficulty. Participants were least satisfied with important tasks that they perceived as highly difficult. CONCLUSION: The results of the present study indicate that not being able to perform valued tasks produces discontent that is reflected in lower rating of satisfaction with physical functioning. Clearly, the significance of loss in function to individual patients is related to the importance of the functional activities that may be compromised. These data have implications for the scope of patient assessment in clinical care and for the conceptual basis of future research in the area of physical functioning.

Activities of Daily Living↗

PubMed2004

Recording spikes from a large fraction of the ganglion cells in a retinal patch.

To understand a neural circuit completely requires simultaneous recording from most of the neurons in that circuit. Here we report recording and spike sorting techniques that enable us to record from all or nearly all of the ganglion cells in a patch of the retina. With a dense multi-electrode array, each ganglion cell produces a unique pattern of activity on many electrodes when it fires an action potential. Signals from all of the electrodes are combined with an iterative spike sorting algorithm to resolve ambiguities arising from overlapping spike waveforms. We verify that we are recording from a large fraction of ganglion cells over the array by labeling the ganglion cells with a retrogradely transported dye and by comparing the number of labeled and recorded cells. Using these methods, we show that about 60 receptive fields of ganglion cells cover each point in visual space in the salamander, consistent with anatomical findings.

Action Potentials↗

PubMed2003

Synergy, redundancy, and independence in population codes.

A key issue in understanding the neural code for an ensemble of neurons is the nature and strength of correlations between neurons and how these correlations are related to the stimulus. The issue is complicated by the fact that there is not a single notion of independence or lack of correlation. We distinguish three kinds: (1) activity independence; (2) conditional independence; and (3) information independence. Each notion is related to an information measure: the information between cells, the information between cells given the stimulus, and the synergy of cells about the stimulus, respectively. We show that these measures form an interrelated framework for evaluating contributions of signal and noise correlations to the joint information conveyed about the stimulus and that at least two of the three measures must be calculated to characterize a population code. This framework is compared with others recently proposed in the literature. In addition, we distinguish questions about how information is encoded by a population of neurons from how that information can be decoded. Although information theory is natural and powerful for questions of encoding, it is not sufficient for characterizing the process of decoding. Decoding fundamentally requires an error measure that quantifies the importance of the deviations of estimated stimuli from actual stimuli. Because there is no a priori choice of error measure, questions about decoding cannot be put on the same level of generality as for encoding.

Action Potentials↗

PubMed2003

Network information and connected correlations.

Entropy and information provide natural measures of correlation among elements in a network. We construct here the information theoretic analog of connected correlation functions: irreducible N-point correlation is measured by a decrease in entropy for the joint distribution of N variables relative to the maximum entropy allowed by all the observed N-1 variable distributions. We calculate the "connected information" terms for several examples and show that it also enables the decomposition of the information that is carried by a population of elements about an outside source.

Information Services↗

PubMed2002

A test of metabolically efficient coding in the retina.

We tested the hypothesis that aspects of the neural code of retinal ganglion cells are optimized to transmit visual information at minimal metabolic cost. Under a broad ensemble of light patterns, ganglion cell spike trains consisted of sparse, precise bursts of spikes. These bursts were viewed as independent neural symbols. The noise in each burst was measured via repeated presentation of the visual stimulus, and the energy cost was estimated from the total charge flow during ganglion cell spiking. Given these costs and noise, the theory of efficient codes predicts an optimal distribution of symbol usage. Symbols that are either noisy or costly occur less frequently in this optimal code. We found good qualitative and quantitative agreement with the measured distribution of burst sizes for ganglion cells in the tiger salamander retina.

Action Potentials↗

PubMedDate not supplied

The metabolic demand of golf in patients with heart disease and in healthy adults.

PURPOSE: To evaluate the metabolic cost of golf, while pulling a cart, in a group of patients with heart disease (HD) and healthy adults with a wide range of functional capacities. METHODS: Twenty male golfers aged 49 to 78 years participated in this study. All participants underwent a graded exercise test (GXT) with expired gas analysis to determine functional capacity. Each patient with HD (n = 10) was matched with a healthy adult of similar age. Each pair completed 9 holes of golf while pulling a cart, during which oxygen consumption was monitored continuously via the Cosmed K4b2 portable unit. RESULTS: The average metabolic equivalent (MET) value (1 MET = 3.5 mL x kg(-1) x min(-1)) (mean +/- SEM) for 9 holes of golf in this group of men with HD (4.1 +/- 0.1 METs) was similar to that previously reported value of 4.3 METs. Whereas the average MET responses were similar between the groups, when expressed relative to peak oxygen consumption, on average, patients with HD worked at a significantly higher percentage of their functional capacity (57 +/- 2.7%) compared to the healthy adults (46 +/- 2.6%). Some patients with HD exceeded 100% of GXT MET level during golf. In contrast, some healthy adults failed to reach 60% of GXT MET level during golf. CONCLUSIONS: Based on these data, walking the golf course while pulling a cart appears to provide an adequate training stimulus for most patients with HD. However, some lower fit patients with HD (< 8 METs) are in danger of exceeding a safe level and should be encouraged to monitor intensity on the golf course and consider using a motorized cart. For most individuals who are more fit (functional capacity > or = 8 METs), golf does not appear to provide the stimulus generally associated with improvement in functional capacity.

Aged↗

PubMedDate not supplied

A randomized, controlled trial comparing long-term and short-term exercise in patients with chronic obstructive pulmonary disease.

PURPOSE: To compare the effects of short-term (3 months) and long-term (18 months) involvement in an exercise program on self-reported disability and physical function in patients with chronic obstructive pulmonary disease (COPD). METHODS: A total of 140 patients with COPD were studied in a randomized, single-blinded clinical trial. Self-reported disability and physical function were assessed using a 21-item questionnaire, a 6-minute walk, timed stair climb, and an overhead task. RESULTS: At the completion of the trial, participants in the long-term intervention reported 12% less disability than those in the short-term intervention (adjusted mean with 95% confidence interval, 1.53 (1.43-1.63) versus 1.71 (1.61 to 1.81) units, respectively; P=.016), walked 6% farther during 6-minutes (1,815.0 [1,750.4-1,879.6] vs 1,711.5 [1,640.7-1,782.3] feet, respectively), climbed steps 11% faster (11.6 [11.0-12.2] vs 12.9 [12.3-13.5] seconds, respectively), and completed an overhead task 8% faster (46.8 [44.4-49.2] vs 50.4 [47.8-53.0] seconds, respectively) than those in the short-term intervention. CONCLUSION: An 18 month exercise program results in greater improvements in self-reported disability and physical function in patients with COPD when compared with a 3-month exercise program. As such, long-term exercise should be recommended for all patients with COPD.

Age Factors↗