Chenchal R Subraveti | Vanderbilt University (original) (raw)

Papers by Chenchal R Subraveti

Journal of Neuroscience, 2010

The role of spike rate versus timing codes in visual target selection is unclear. We simultaneous... more The role of spike rate versus timing codes in visual target selection is unclear. We simultaneously recorded activity from multiple frontal eye field neurons and asked whether they interacted to select targets from distractors during visual search. When both neurons in a pair selected the target and had overlapping receptive fields (RFs), they cooperated more than when one or neither neuron in the pair selected the target, measured by positive spike timing correlations using joint peristimulus time histogram analysis. The amount of cooperation depended on the location of the search target: it was higher when the target was inside both neurons' RFs than when it was inside one RF but not the other, or outside both RFs. Elevated spike timing coincidences occurred at the time of attentional selection of the target as measured by average modulation of discharge rates. We observed competition among neurons with spatially non-overlapping RFs, measured by negative spike timing correlations. Thus, we provide evidence for dynamic and task-dependent cooperation and competition among frontal eye field neurons during visual target selection.

Journal of Neurophysiology, 2007

The frontal eye field (FEF) is involved in selecting visual targets for eye movements. To underst... more The frontal eye field (FEF) is involved in selecting visual targets for eye movements. To understand how populations of FEF neurons interact during target selection, we recorded activity from multiple neurons simultaneously while macaques performed two versions of a visual search task. We used a multivariate analysis in a point process statistical framework to estimate the instantaneous firing rate and compare interactions among neurons between tasks. We found that FEF neurons were engaged in more interactions during easier visual search tasks compared with harder search tasks. In particular, eye movement–related neurons were involved in more interactions than visual-related neurons. In addition, our analysis revealed a decrease in the variability of spiking activity in the FEF beginning ∼100 ms before saccade onset. The minimum in response variability occurred ∼20 ms earlier for the easier search task compared with the harder one. This difference is positively correlated with the d...

The Journal of Neuroscience, 2001

Although many studies have explored the neural correlates of visual attention and selection, few ... more Although many studies have explored the neural correlates of visual attention and selection, few have examined the reliability with which neurons represent relevant information. We monitored activity in the frontal eye field (FEF) of monkeys trained to make a saccade to a target defined by the conjunction of color and shape or to a target defined by color differences. The difficulty of conjunction search was manipulated by varying the number of distractors, and the difficulty of feature search was manipulated by varying the similarity in color between target and distractors. The reliability of individual neurons in signaling the target location in correct trials was determined using a neuron-anti-neuron approach within a winner-take-all architecture. On average, approximately seven trials of the activity of single neurons were sufficient to match near-perfect behavioral performance in the easiest search, and ϳ14 trials were sufficient in the most difficult search. We also determined how many neurons recorded separately need to be evaluated within a trial to match behavioral performance. Results were quantitatively similar to those of the single neuron analysis. We also found that signal reliability in the FEF did not change with task demands, and overall, behavioral accuracy across the search tasks was approximated when only six trials or neurons were combined. Furthermore, whether combining trials or neurons, the increase in time of target discrimination corresponded to the increase in mean saccade latency across visual search difficulty levels. Finally, the variance of spike counts in the FEF increased as a function of the mean spike count, and the parameters of this relationship did not change with attentional selection.

Numerous studies have described different functional cell types in the frontal eye field (FEF), b... more Numerous studies have described different functional cell types in the frontal eye field (FEF), but the reliability of the distinction between these types has been uncertain. Studies in other brain areas have described specific differences in the width of action potentials recorded from different cell types. To substantiate the functionally defined cell types encountered in FEF, we measured the width of spikes of visual, movement, and visuomovement types of FEF neurons in macaque monkeys. We show that visuomovement neurons had the thinnest spikes, consistent with a role in local processing. Movement neurons had the widest spikes, consistent with their role in sending eye movement commands to subcortical structures such as the superior colliculus. Visual neurons had wider spikes than visuomovement neurons, consistent with their role in receiving projections from occipital and parietal cortex. These results show how structure and function of FEF can be linked to guide inferences about neuronal architecture. Local circuit neurons immunoreactive for calretinin, calbindin D-28k or parvalbumin in monkey prefrontal cortex: distribution and morphology. J Comp Neurol 341: 95-116, 1994. Connors BW, Gutnick MJ. Intrinsic firing patterns of diverse neocortical neurons. Trends Neurosci 13: 99 -104, 1990. Constantinidis C, Goldman-Rakic PS. Correlated discharges among putative pyramidal neurons and interneurons in the primate prefrontal cortex. J Neurophysiol 88: 3487-3497, 2002. Csicsvari J, Hirase H, Czurkó A, Mamiya A, Buzsáki G. Oscillatory coupling of hippocampal pyramidal cells and interneurons in the behaving rat. J Neurosci 19: 274 -287, 1999. DiCarlo JJ, Maunsell JH. Using neuronal latency to determine sensory-motor processing pathways in reaction time tasks. J Neurophysiol 93: 2974 -2986, 2005. Dow BM. Functional classes of cells and their laminar distribution in monkey visual cortex. J Neurophysiol 37: 927-946, 1974. Fries W. Cortical projections to the superior colliculus in the macaque monkey: a retrograde study using horseradish peroxidase. J Comp Neurol 230: 55-76, 1984. Goldberg ME, Bushnell MC. Behavioral enhancement of visual responses in monkey cerebral cortex. II. Modulation in frontal eye fields specifically related to saccades. J Neurophysiol 46: 773-787, 1981. González-Burgos G, Krimer LS, Povysheva NV, Barrionuevo G, Lewis DA. Functional properties of fast spiking interneurons and their synaptic connections with pyramidal cells in primate dorsolateral prefrontal cortex. J Neurophysiol 93: 942-953, 2005. Gur M, Beylin A, Snodderly DM. Physiological properties of macaque V1

Journal of Neurophysiology, 2007

You might find this additional info useful... for this article has been published. It can be foun... more You might find this additional info useful... for this article has been published. It can be found at: A corrigendum /content/99/2/1050.full.html 30 articles, 17 of which can be accessed free at: This article cites /content/98/5/2580.full.html#ref-list-1 5 other HighWire hosted articles This article has been cited by [PDF] [Full Text] [Abstract] 2009; 101 (2): 912-916. J Neurophysiol [PDF] [Full Text] [Abstract]

Neuropsychologia, 2001

A central issue in mental chronometry is whether information is transferred between processing st... more A central issue in mental chronometry is whether information is transferred between processing stages such as stimulus evaluation and response preparation in a continuous or discrete manner. We tested whether partial information about a stimulus influences the response stage by recording the activity of movement-related neurons in the frontal eye field of macaque monkeys performing a conjunction visual search and a feature visual search with a singleton distractor. While movement-related neurons were activated maximally when the target of the search array was in their movement field, they were also activated for distractors even though a saccade was successfully made to the target outside the movement field. Most importantly, the level of activation depended on the properties of the distractor, with greater activation for distractors that shared a target feature or were the target during the previous session during conjunction search, and for the singleton distractor during feature search. These results support the model of continuous information processing and argue against a strictly discrete model.

We examined neural activity in prefrontal (PF) cortex of monkeys performing a delayed paired asso... more We examined neural activity in prefrontal (PF) cortex of monkeys performing a delayed paired associate task. Monkeys were cued with a sample object. Then, after a delay, a test object was presented. If the test object was the object associated with the sample during training (i.e., its target), they had to release a lever. Monkeys could bridge the delay by remembering the sample (a sensory-related code) and/or thinking ahead to the expected target (a prospective code). Examination of the monkeys' behavior suggested that they were relying on a prospective code. During and shortly after sample presentation, neural activity in the lateral PF cortex primarily reflected the sample. Toward the end of the delay, however, PF activity began to reflect the anticipated target, which indicated a prospective code. These results provide further confirmation that PF cortex does not simply buffer incoming visual inputs, but instead selectively processes information relevant to current behavioral demands, even when this information must be recalled from long-term memory.

Journal of Comparative Neurology, 1997

Feature maps in the cerebral cortex constitute orderly representations of response features creat... more Feature maps in the cerebral cortex constitute orderly representations of response features created within the cortex; an example is the mapping of orientation-selective neurons in visual cortex. We have compared the properties of orientation maps in area 17 of cats and ferrets, obtained by optical imaging of intrinsic signals. Orientation maps in both species contain a quasi-periodic distribution of iso-orientation domains that are organized into a lattice of pinwheels. However, the spatial density of orientation domains and of pinwheels in ferret area 17 is nearly twice that in cat area 17. The ferret map also contains more discontinuities, or fractures, where orientation changes abruptly. The size of orientation domains scales with interdomain spacing, so that the ratio of the two is approximately the same in both species. Consistent with this finding, the orientation tuning width of individual pixels is similar in the two. The magnitude of orientation preference, however, is much lower in ferret compared to cat. The greater incidence of fractures in ferret appears to be due to proportionately greater overlap between domains of different orientations, particularly along fracture lines that link pinwheel centers. We hypothesize that a key determinant of orientation maps, the relationship between orientation domain size and spacing, expresses an anatomical link between sizes of thalamocortical arbors and horizontal intracortical connections in area 17.

Science, 1996

6. Escherichia coli ENR is a homotetramer (M r ϳ 28,000 per subunit) that was prepared from an ov... more 6. Escherichia coli ENR is a homotetramer (M r ϳ 28,000 per subunit) that was prepared from an overexpressing E. coli strain (2, 5). Crystals of the ENR-NAD ϩ complex (crystal form A) belong to space group P2 1 and have unit cell dimensions of a ϭ 74.0 Å, b ϭ 81.2 Å, c ϭ 79.0 Å, and ␤ ϭ 92.9°with a tetramer in the asymmetric unit (16). Data were collected to 2.5 Å ( , data set Native-1) on a twin San Diego Multiwire Systems (SDMS) area detector with a Rigaku RU-200 rotating anode source, and the data set was processed with SDMS software (17 ). Data were also collected to 2.1 Å , data set Native-2) at the CLRC Daresbury Synchrotron and processed with the MOSFLM package (18), and the 2.1 and 2.5 Å data sets were then scaled and merged with CCP4 software (19). Initially, a model of B. napus ENR (10) was used as a basis for a molecular replacement solution of the structure, but the map, calculated after the model was refined with the program TNT (20), was not of sufficient quality to confidently assign residues in regions of structural differences between the B. napus and E. coli enzymes. Therefore, to solve the structure, we obtained a heavy-atom derivative by soaking an ENR-NAD ϩ (form A) crystal for 1 hour in 0.1 mM ethylmercuriphosphate, 10 mM NAD ϩ , 20% (w/v) polyethylene glycol (molecular weight 400), and 100 mM acetate (pH 5.0). Derivative data were collected at the CLRC Daresbury Synchrotron to a resolution of 3 Å , data set Hg) and were processed as above. The positions of the heavy atoms in this derivative were revealed by difference Fourier methods with the use of the approximate phases provided by the molecular replacement solution. The heavy-atom parameters were refined with the program MLPHARE (21) and resulted in a phase set with an overall mean figure of merit of 0.34 to 3 Å resolution. Using a map derived from these phases, we generated molecular masks for the molecule with the program MAMA (22) and performed 50 cycles of solvent flattening and fourfold molecular averaging with the program DM (19, 23). In the resultant electron density map, calculated from the averaged phases, we were able to find clear density for all but the first residue, the last four residues, and 10 residues from the loop joining ␤6 and ␣6; using the graphics program FRODO (24), we were able to build with confidence a model comprising 247 of the 262 amino acids of E. coli ENR. Several cycles of rebuilding and refinement gave a final R factor for the model of 0.157 (52,346 reflections in the range 10 to 2.1 Å, 7836 atoms including 324 water molecules), with an rmsd of 0.017 Å for bonds and 2.92°for angles [R ϭ ⌺(FobsϪF calc )/⌺(F obs ), where F obs  and F calc  are the observed and calculated structure factor amplitudes, respectively]. The average B factor for the tetramer is 30 Å 2 (24 Å 2 for main-chain atoms), where B ϭ 8 2 ( 2 ) and is the mean square displacement of the atomic vibration. 7. Crystals of the ENR-NAD ϩ -diazaborine complex (crystal form B) belong to space group P6 1 22 and have unit cell dimensions of a ϭ b ϭ 80.9 Å, c ϭ 328.3 Å, ␣ ϭ ␤ ϭ 90°, and ␥ ϭ 120°for the thienodiazaborine complex, and a ϭ b ϭ 80.6 Å, c ϭ 325.3 Å, ␣ ϭ ␤ ϭ 90°, and ␥ ϭ 120°for the benzodiazaborine complex with a dimer in the asymmetric unit (16). Data sets were collected on the ENR-NAD ϩthienodiazaborine complex to 2.2 Å and on the ENR-NAD ϩ -benzodiazaborine complex to 2.5 Å , data sets Thieno and Benzo) at the CLRC Daresbury Synchrotron and were processed as above. The structures of both ENR-NAD ϩ -diazaborine complexes were solved independently by molecular replacement with the use of an appropriate dimer from the E. coli ENR-NAD ϩ structure and were refined against their respective data sets with the program TNT (20). The initial electron density maps were readily interpretable and unambiguous density could be observed for the location of the diazaborine compounds, which were then incorporated into the refinement. Clear density could be found for all but the first residue and the last four residues. Refinement of the thienodiazaborine complex gave a final R factor of 0.191 (30,825 reflections in the range 10 to 2.2 Å, 3936 atoms), with an rmsd of 0.012 Å for bonds and 2.9°for angles. The average B factor for the dimer is 27 Å 2 (22 Å 2 for main-chain atoms, 20 Å 2 for diazaborine atoms). Refinement of the benzodiazaborine complex gave a final R factor of 0.169 (20,204 reflections in the range 10 to 2.5 Å, 3930 atoms), with an rmsd of 0.013 Å for bonds and 2.7°f or angles. The average B factor for the dimer is 24 Å 2 (20 Å 2 for main-chain atoms, 20 Å 2 for diazaborine atoms). For the ENR-NAD ϩ complex and the ENR-NAD ϩ -thienodiazaborine complex, 244 C␣ atoms superimpose with an rmsd of 0.3 Å, whereas for the two ENR-NAD ϩ -diazaborine complexes, 256 C␣ atoms superimpose with an rmsd of 0.2 Å.

Cerebral Cortex, 1997

In mammalian visual cortex, local connections are ubiquitous, extensively linking adjacent neuron... more In mammalian visual cortex, local connections are ubiquitous, extensively linking adjacent neurons of all types. In this study, optical maps of intrinsic signals and responses from single neurons were obtained from the same region of cat visual cortex while the effectiveness of the local cortical circuitry was altered by focally disinhibiting neurons within a column of known orientation preference. Maps of intrinsic signals indicated that local connections provide strong and functional subthreshold inputs to neighboring columns of other orientation preferences, altering the observed orientation preference to that of the disinhibited column. However, measuring the suprathreshold response using single-cell recordings revealed only mild changes of preferred orientation over the affected region. Because strongly tuned subthreshold inputs from cortex only marginally affect the tuning of a cortical cell's output, it is concluded that local cortical inputs are integrated weakly compared to geniculate inputs. Such circuitry potentially allows for the normalization of responses across a wide range of input activity through local averaging.

Proceedings of The National Academy of Sciences, 1996

Neurons in primary visual cortex (area 17) respond vigorously to oriented stimuli within their re... more Neurons in primary visual cortex (area 17) respond vigorously to oriented stimuli within their receptive fields; however, stimuli presented outside the suprathreshold receptive field can also influence their responses. Here we describe a fundamental feature of the spatial interaction between suprathreshold center and subthreshold surround. By optical imaging of intrinsic signals in area 17 in response to a stimulus border, we show that a given stimulus generates activity primarily in iso-orientation domains, which extend for several millimeters across the cortical surface in a manner consistent with the architecture of long-range horizontal connections in area 17. By mapping the receptive fields of single neurons and imaging responses from the same cortex to stimuli that include or exclude the aggregate suprathreshold receptive field, we show that intrinsic signals strongly reveal the subthreshold surround contribution. Optical imaging and single-unit recording both demonstrate that the relative contrast of center and surround stimuli regulates whether surround interactions are facilitative or suppressive; the same surround stimulus facilitates responses when center contrast is low, but suppresses responses when center contrast is high. Such spatial interactions in area 17 are ideally suited to contribute to phenomena commonly regarded as part of ``higher-level'' visual processing, such as perceptual ``popout'' and ``filling-in.''

Photochemistry and Photobiology, 1989

Shift in the wavelength of emission upon shift in the excitation wavelength towards the red edge ... more Shift in the wavelength of emission upon shift in the excitation wavelength towards the red edge of the absorption band is termed Red Edge Excitation Shift (REES). This effect is observed only in a situation where the fluorophore mobility with respect to the surrounding matrix is considerably reduced. We have observed such red edge excitation effect in the intact eye lens. The REES observed for a normal lens is different from that seen in a photodamaged lens and hence appears to be a potential tool to monitor the changes in the state of the lens. Photodamage experiments with tryptophan in polyethylene glycol (PEG) and intact eye lens indicate that the red edge photon can also cause photodamage.

Journal of Chemical Sciences, 1994

Proteins that perform other functions elsewhere appear to be recruited for structural purposes in... more Proteins that perform other functions elsewhere appear to be recruited for structural purposes in the eye lens. The lens being a tissue with very little metabolic activity and little or no turnover, the lens proteins, crystallins, are long lived. In an effort to understand whether their recruitment might be related to their conformation and structural stability, we have examined these features of the avian lens protein δ-crystallin. The native molecule is a tetramer (molecular mass 200 kDa) that is highly α-helical in conformation, and with an unusually blue tryptophan fluorescence (315,325 nm), which is only partially quenched by conventional quenchers. We show that the fluorescence doublet arises due to Trp residues that are effectively buried inside the rigid hydrophobic core of the tetrameric aggregate. The protein is heat stable up to 91°C. Guanidinium chloride (GuHCl) effects the complete denaturation of δ-crystallin, whereas heat or urea treatment results in only partial unfolding or dissociation. The initial transition is the disruption of the quaternary structure by perturbing the intersubunit interactions, leading to exposure of hydrophobic contact surfaces (as monitored by extrinsic probe fluorescence). This initial transition is seen upon heating to 60°C as well as in 1 M GuHCl and 4 M urea. We show that in 2.2 M GuHCl the molecule is swollen but is still largely helical with the Trp residues being present in a somewhat more polar environment than in the native molecule. Beyond 4 M GuHCl there is a gradual unfolding of the molecule, which is complete in 6 M GuHCl. This structural robustness of δ-crystallin might be important in its recruitment as the core protein of the avian lens.

Photochemistry and Photobiology, 1990

Abstract— We have studied the aerobic photolysis of the tryptophan residues of the proteins melit... more Abstract— We have studied the aerobic photolysis of the tryptophan residues of the proteins melittin and p-lactoglobulin when the proteins are in ordered conformations and when they are in randomly coiled states. The results suggest that the conformational status of the protein is a factor that influences the photolysis of the constituent tryptophan residues. This point appears to be of relevance to the photo-oxidation of the tryptophan residues of the eye lens proteins crystallins.

Abnormal levels of endogenous calcium ions are known to induce eye lens opacity, and a variety of... more Abnormal levels of endogenous calcium ions are known to induce eye lens opacity, and a variety of causative factors has been proposed, including calcium-mediated aggregation and precipitation of the lens proteins crystallins. We have specifically looked in some detail at the interaction of Ca" with various crystallins and its consequences. Lenses incubated in solutions containing 10 m M ca2+ or 5 m M Tb3+ opacified. Fluorescence titration of crystallins with TbCls revealed that this ion binds to 6-and B-crystallins in solution. Equilibrium dialysis showed that four Ca" ions bind to one &crystallin tetramer with an affinity of 4.3 X los M-'. Analysis of the amino acid sequence of 6-crystallin reveals the presence of a calmodulintype "helix-loop-helix" or "EF-hand" calcium ion binding conformational motif in the region comprising residues 300-350. This is a novel feature of the molecule not reported so far. No other crystallins appear to have this motif. &Crystallin also binds four Ca2+ ionslaggregate unit of mass 160 kDa, with an affinity of 2.6 X lo3 M-', presumably in the midregion of the molecule that is rich in anionic and polar residues. Circular dichroism spectroscopy shows that the binding of calcium ion leads to subtle conformational changes in the molecules, notably in the tertiary structure. Biochim. Biophys. Acta 824,295-303 Commun. 128,1322L1328 Photochem. Photobiol. 47,583-591 ~ ~I~ , ~ I~ ~.-" -""

Science, 1996

Responses to subjective contours in visual cortical areas V1 and V2 in adult cats were investigat... more Responses to subjective contours in visual cortical areas V1 and V2 in adult cats were investigated by optical imaging of intrinsic signals and single-unit recording. Both V1 and V2 contain maps of the orientation of subjective gratings that have their basis in specific kinds of neuronal responses to subjective orientations. A greater proportion of neurons in V2 than in V1 show a robust response to subjective edges. Through the use of subjective stimuli in which the orientation of the luminance component is invariant, an unmasked V1 response to subjective edges alone can be demonstrated. The data indicate that the processing of subjective contours begins as early as V1 and continues progressively in higher cortical areas.

Journal of Neuroscience, 2010

Journal of Neurophysiology, 2007

The Journal of Neuroscience, 2001

Journal of Neurophysiology, 2007

Neuropsychologia, 2001

Journal of Comparative Neurology, 1997

Science, 1996

Cerebral Cortex, 1997

Proceedings of The National Academy of Sciences, 1996

Photochemistry and Photobiology, 1989

Journal of Chemical Sciences, 1994

Photochemistry and Photobiology, 1990

Science, 1996