Biomedical subjects
Publications and source records attributed to Daniel J Simons.
Rhythmic whisker movements, called "whisking," are produced by a brainstem central pattern generator (CPG) that uses serotonin to induce periodic firing in facial motorneurons. During active touch, motor cortex could regulate whisking frequency by controlling the rate of firing of the serotonergic neurons.
Inhibitory neurons of the thalamic reticular (RT) nucleus fire in two activity modes, burst and tonic, depending on an animal's behavioral state. In tonic mode, depolarized RT cells fire single action potentials continuously, whereas burst firing consists of grouped discharges separated by periods of quiescence. To determine how these firing modes affect sensory-evoked RT responses, single-unit responses to controlled whisker deflections were analyzed according to the burst versus tonic mode of spontaneous activity (SA) preceding the response. After burst mode activity (i.e., either quiescence or spontaneous bursts), responses exhibited a slow approximately 15 msec rise to peak firing rates followed by a approximately 35 msec decay. Interspike intervals within the response exhibited accelerando-decelerando patterns similar to those of spontaneous bursts. After tonic mode activity (i.e., single spikes), responses had a nearly instantaneous approximately 1.5 msec rise-to-peak followed by a approximately 40 msec decay, with large spike counts (5.2 spikes per stimulus) similar to those evoked in burst mode (6.2 spikes per stimulus). Interspike intervals were longer in tonic mode and exhibited a decelerando pattern. Initial evoked spikes, however, had shorter latencies and greater synchrony, contributing to the rapid onset of tonic population response. Shifts from quiescent (presumed burst mode) to tonic SA could be induced by either previous whisker deflections or iontophoretic application of NMDA; both manipulations effected appropriate shifts from burst to tonic response spike patterns. In awake animals, burst and tonic firing in RT, as in thalamocortical relay nuclei, may reflect sensory processing strategies appropriate for different behavioral and attentional states.
The rodent whisker-to-barrel pathway constitutes a major model system for studying experience-dependent brain development. Yet little is known about responses of neurons to whisker stimulation in young animals. Response properties of trigeminal ganglion (NV) neurons in 2-, 3-, and 4-week-old and adult rats were examined using extracellular single-unit recordings and controlled whisker stimuli. We found that the receptive field size of NV neurons is mature in 2-week-old animals while response latencies, magnitudes, and angular tuning continue to develop between 2 weeks of age and adulthood. At the earliest time recorded, NV neurons respond to stimulation of only one whisker and can be characterized as slowly or rapidly adapting (SA, RA). The proportion of SA and RA neurons remains constant during development. Consistent with known on-going myelination of NV axons, response latencies decrease with age, becoming adult-like during the third and fourth postnatal weeks for RA and SA neurons, respectively. Unexpectedly, we found that evoked response magnitudes increase several-fold during development becoming adult-like only during the fourth postnatal week. In addition, RA neurons become less selective for whisker deflection angle with age. Maturation of response magnitude and angular tuning is consistent with developmental changes in the mechanical properties of the whisker, the whisker follicle, and the surrounding tissues. The findings indicate that whisker-derived tactile inputs mature during the first postnatal month when whisker-related cortical circuits are susceptible to long-term modification by sensory experience. Thus normal developmental changes in sensory input may influence functional development of cortical circuits.
To understand how the lemniscal trigeminothalamic circuit (PrV --> VPM) of the rodent whisker-to-barrel pathway transforms afferent signals, we applied ramp-and-hold deflections to individual whiskers of lightly narcotized rats while recording the extracellular responses of neurons in either the ventroposterior medial (VPM) thalamic nucleus or in brain stem nucleus principalis (PrV). In PrV, only those neurons antidromically determined to project to VPM were selected for recording. We found that VPM neurons exhibited smaller response magnitudes and greater spontaneous firing rates than those of their PrV inputs, but that both populations were similarly well tuned for stimulus direction. In addition, fewer VPM (74%) than PrV neurons (93%) responded with sustained, or tonic, discharges during the plateau phase of the stimulus. Neurons in both populations responded most robustly to deflections of a single, "principal whisker" (PW), and the majority of cells in both PrV (90%) and VPM (73%) also responded to deflections of at least one adjacent whisker (AW). AW responses in both nuclei occurred on average at longer latencies and were more temporally dispersed than PW responses. Lateral inhibition, as evidenced by AW-evoked activity suppression, was rare in PrV but prevalent in VPM. In both nuclei, however, suppression was weak, with AW responses being on average excitatory. Our results suggest that the receptive-field structures and response properties of individual VPM neurons can be explained in large part by input from one or a small number of PrV neurons, but that intrathalamic mechanisms act to further transform the afferent signal.
Observers typically detect changes to central objects more readily than changes to marginal objects, but they sometimes miss changes to central, attended objects as well. However, even if observers do not report such changes, they may be able to recognize the changed object. In three experiments we explored change detection and recognition memory for several types of changes to central objects in motion pictures. Observers who failed to detect a change still performed at above chance levels on a recognition task in almost all conditions. In addition, observers who detected the change were no more accurate in their recognition than those who did not detect the change. Despite large differences in the detectability of changes across conditions, those observers who missed the change did not vary in their ability to recognize the changing object.
Whisker plucking in developing and adult rats provides a convenient method of temporarily altering tactile input for the purposes of studying experience-dependent plasticity in the somatosensory cortex. Yet, a comprehensive examination of the effect of whisker plucking on the response properties of whisker follicle-innervating trigeminal ganglion (NVg) neurons is lacking. We used extracellular single unit recordings to examine responses of NVg neurons to controlled whisker stimuli in three groups of animals: (1) rats whose whiskers were plucked from birth for 21 days; (2) rats whose whiskers were plucked once at 21 days of age; and (3) control animals. After at least 3 weeks of whisker re-growth, NVg neurons in plucked rats displayed normal, single whisker receptive fields and could be characterized as slowly (SA) or rapidly adapting (RA). The proportion of SA and RA neurons was unaffected by whisker plucking. Both SA and RA NVg neurons in plucked rats displayed normal response latencies and angular tuning but abnormally large responses to whisker movement onsets and offsets. SA neurons were affected to a greater extent than RA neurons. The effect of whisker plucking was more pronounced in animals whose whiskers were plucked repeatedly during development than in rats whose whiskers were plucked once. Individual neurons in plucked animals displayed abnormal periods of prolonged rhythmic firing following deflection onsets and aberrant bursts of activity during the plateau phase of the stimulus. These results indicate that whisker plucking exerts a long-term effect on responses of trigeminal ganglion neurons to peripheral stimulation.
In the whisker-barrel system, layer IV excitatory neurons respond preferentially to high-velocity deflections of their principal whisker, and these responses are inhibited by deflections of adjacent whiskers. Thalamic input neurons are amplitude and velocity sensitive and have larger excitatory and weaker inhibitory receptive fields than cortical neurons. Computational models based on known features of barrel circuitry capture these and other differences between thalamic and cortical neuron response properties. The models' responses are highly sensitive to thalamic firing synchrony, a finding subsequently confirmed in real barrels by in vivo experiments. Here, we use dynamic systems analysis to examine how barrel circuitry attains its sensitivity to input timing, and how this sensitivity explains the transformation of receptive fields between thalamus and cortex. We find that strong inhibition renders the net effect of intracortical connections suppressive or damping, distinguishing it from previous amplifying models of cortical microcircuits. In damping circuits, recurrent excitation enhances response tuning not by amplifying responses to preferred inputs, but by enabling them to better withstand strong inhibitory influences. Dense interconnections among barrel neurons result in considerable response homogeneity. Neurons outside the barrel layer respond more heterogeneously, possibly reflecting diverse networks and multiple transformations within the cortical output layers.
Nucleus principalis (PrV) of the brain stem trigeminal complex mediates the processing and transfer of low-threshold mechanoreceptor input en route to the ventroposterior medial nucleus of the thalamus (VPM). In rats, this includes tactile information relayed from the large facial whiskers via primary afferent fibers originating in the trigeminal ganglion (NV). Here we describe the responses of antidromically identified VPM-projecting PrV neurons (n = 72) to controlled ramp-and-hold deflections of whiskers. For comparison, we also recorded the responses of 64 NV neurons under identical experimental and stimulus conditions. Both PrV and NV neurons responded transiently to stimulus onset (ON) and offset (OFF), and the majority of both populations also displayed sustained, or tonic, responses throughout the plateau phase of the stimulus (75% of NV cells and 93% of PrV cells). Average ON and OFF response magnitudes were similar between the two populations. In both NV and PrV, cells were highly sensitive to the direction of whisker deflection. Directional tuning was slightly but significantly greater in NV, suggesting that PrV neurons integrate inputs from NV cells differing in their preferred directions. Receptive fields of PrV neurons were typically dominated by a "principal" whisker (PW), whose evoked responses were on average threefold larger than those elicited by any given adjacent whisker (AW; n = 197). However, of the 65 PrV cells for which data from at least two AWs were obtained, most (89%) displayed statistically significant ON responses to deflections of one or more AWs. AW response latencies were 2.7 +/- 3.8 (SD) ms longer than those of their corresponding PWs, with an inner quartile latency difference of 1-4 ms (+/-25% of median). The range in latency differences suggests that some adjacent whisker responses arise within PrV itself, whereas others have a longer, multi-synaptic origin, possibly via the spinal trigeminal nucleus. Overall, our findings reveal that the stimulus features encoded by primary afferent neurons are reflected in the responses of VPM-projecting PrV neurons, and that significant convergence of information from multiple whiskers occurs at the first synaptic station in the whisker-to-barrel pathway.
Rats employ rhythmic whisker movements to sample information in their sensory environment. To study frequency tuning and filtering characteristics of thalamic circuitry, we recorded single-unit responses of ventroposterior medial (VPm) and thalamic reticular (Rt) neurons to 1- to 40-Hz sinusoidal and pulsatile whisker deflection in lightly narcotized rats. Neuronal entrainment was assessed by a measure of the relative modulation (RM) of firing at the stimulus frequency given by the first harmonic (F1) of the cycle time histogram divided by the mean firing rate (F0). VPm signaling of both sinusoidal and periodic pulsatile whisker movements improved gradually over 1-16 and was maximal at 20-40 Hz. By contrast, the RM of Rt responses increased over 1-8 Hz, but deteriorated progressively over the 12- to 40-Hz range. In Rt, response adaptation occurred at lower stimulus frequencies and to a greater extent than in VPm. Within a train of high-frequency stimuli, Rt responses progressively decremented, possibly due to the accumulation of inhibition, whereas those of VPm neurons augmented. Mean firing rates in Rt increased 42 spikes/s over 1-40 Hz, providing tonic (low RM) inhibition during high-frequency stimulation that may enhance VPm signal-to-noise ratios. Consistent with this view, VPm mean firing rates increased only 13 spikes/s over 1-40 Hz, and inter-deflection activity was suppressed to a greater extent than stimulus-evoked responses. Rt inhibition is likely to act in concert with actions of neuromodulators in optimizing thalamic temporal signaling of high-frequency whisker movements.
Attention capture is often operationally defined as speeded search performance when an otherwise nonpredictive stimulus happens to be the target of a visual search. That is, if a stimulus captures attention, it should be searched with priority even when it is irrelevant to the task. Given this definition, only the abrupt appearance of a new object (see, e.g., Jonides & Yantis, 1988) and one type of luminance contrast change (Enns, Austen, Di Lollo, Rauschenberger, & Yantis, 2001) have been shown to strongly capture attention. We show that translating and looming stimuli also capture attention. This phenomenon does not occur for all dynamic events: We also show that receding stimuli do not attract attention. Although the sorts of dynamic events that capture attention do not fit neatly into a single category, we speculate that stimuli that signal potentially behaviorally urgent events are more likely to receive attentional priority.
In layer IV of rat somatosensory cortex, barrel circuitry is highly sensitive to thalamic population firing rates during the first few milliseconds of the whisker-evoked response. This sensitivity of barrel neurons to thalamic firing synchrony was inferred previously from analysis of simulated barrel circuitry and from single-unit recordings performed one at a time. In this study, we investigate stimulus-dependent synchronous activity in the thalamic ventral posteromedial nucleus (VPm) using the more direct approach of local field potential (LFP) recording. We report that thalamic barreloid neurons generate larger magnitude LFP responses to principal versus adjacent whiskers, to preferred versus nonpreferred movement directions, and to high- versus low-velocity/acceleration deflections. Responses were better predicted by acceleration than velocity, and they were insensitive to the final amplitude of whisker deflection. Importantly, reliable and robust stimulus/response relationships were found only for the initial 1.2-7.5 ms of the thalamic LFP response, reflecting arrival of afferent information from the brain stem. Later components of the thalamic response, which are likely to coincide with arrival of inhibitory inputs from the thalamic reticular nucleus and excitatory inputs from the barrel cortex itself, are variable and poorly predicted by stimulus parameters. Together with previous results, these findings underscore a critical role for thalamic firing synchrony in the encoding of small but rapidly changing perturbations of specific whiskers in particular directions.
Excitatory and inhibitory cortical layer IV neurons have distinctive response properties. Thalamocortical connectivity that may underlie differences was examined using cross-correlation analyses of pairs of thalamic and cortical neurons in the rat whisker/barrel system. Cortical layer IV cells discharging fast spikes, presumed inhibitory neurons, were distinguished from regular-spike units, presumed excitatory neurons, by the extracellular waveform shape. Regular-spike neurons fired less robustly and had smaller receptive fields (RFs) and greater directional tuning than fast-spike cells. Presumed excitatory neurons were less likely to receive thalamocortical connections, and their connections were, on average, weaker. RF properties of thalamic inputs to both cell types were equivalent, except that the most highly responsive thalamic cells contacted only fast-spike neurons. In contrast, the size and directional tuning of cortical RFs were related to the number of detectable thalamocortical inputs. Connected thalamocortical pairs were likely to have matching RF characteristics. The smaller, more directionally selective RFs of excitatory neurons may be a consequence of their weaker net thalamic drive, their more nonlinear firing characteristics and pervasive feedforward inhibition provided by strongly driven, broadly tuned inhibitory neurons.
People often fail to detect large changes to scenes, provided that the changes occur during a visual disruption. This phenomenon, known as "change blindness," occurs both in the laboratory and in real-world situations in which changes occur unexpectedly. The pervasiveness of the inability to detect changes is consistent with the theoretical notion that we internally represent relatively little information from our visual world from one glance at a scene to the next. However, evidence for change blindness does not necessarily imply the absence of such a representation---people could also miss changes if they fail to compare an existing representation of the pre-change scene to the post-change scene. In three experiments, we show that people often do have a representation of some aspects of the pre-change scene even when they fail to report the change. And, in fact, they appear to "discover" this memory and can explicitly report details of a changed object in response to probing questions. The results of these real-world change detection studies are discussed in the context of broader claims about change blindness.
We have developed a semi-automated technique for acquiring neurophysiological data during whisker-based tactile discriminative behavior. Water-deprived, blindfolded rats are tethered by means of a harness vest that permits them to contact a rough (250 micrometer grooves) or smooth discriminandum with only their vibrissae. Discriminanda are mounted on a motor-driven carousel, and the rat indicates its choice (rough, smooth) by licking either a right or left water port located near the carousel. A narrow light beam detects general proximity of the animal's nose to the discriminandum, although actual whisker contact is monitored by a SuperVHS camera and measured offline using field-by-field videographic analysis. Rats can be trained within 3-6 weeks at which time they perform 100-150 trials/day at a level of 80% correct. Unit recording from the somatosensory cortex reveals that neurons increase their firing upon whisker contact of a discriminandum and that firing remains elevated during several hundred milliseconds of ongoing contact, even with the smooth surface. Nevertheless, despite the animal's ability to distinguish the rough and smooth surfaces, overall neuronal firing rates were indistinguishable for the two surfaces. In some cases, temporal firing patterns differed, although not in a consistent way across recording sites.
People often have difficulty detecting visual changes in scenes, a phenomenon referred to as 'change blindness'. Although change blindness is usually observed in pictures of objects that are not the focus of attention, it also occurs for attended objects in the real world. Here, we further explore the finding that many participants fail to detect the unexpected substitution of one conversation partner for another. We show that change blindness for a conversation partner occurs in a variety of situations. Furthermore, when tested with a photographic lineup following the change, participants who noticed the substitution showed better memory for both pre- and post-change experimenters than participants who did not detect the change. We conclude that change blindness in this case is associated with relatively ineffective or inaccessible representations of previously attended objects, and we contrast these results with others indicating that change blindness arises from a failure to compare the original and changed object.
Many previous studies of object recognition have found view-dependent recognition performance when view changes are produced by rotating objects relative to a stationary viewing position. However, the assumption that an object rotation is equivalent to an observer viewpoint change ignores the potential contribution of extraretinal information that accompanies observer movement. In four experiments, we investigated the role of extraretinal information on real-world object recognition. As in previous studies focusing on the recognition of spatial layouts across view changes, observers performed better in an old/new object recognition task when view changes were caused by viewer movement than when they were caused by object rotation. This difference between viewpoint and orientation changes was due not to the visual background, but to the extraretinal information available during real observer movements. Models of object recognition need to consider other information available to an observer in addition to the retinal projection in order to fully understand object recognition in the real world.
Although change blindness could suggest that observers represent far less of their visual world than their conscious experience leads them to believe, they could fail to detect changes even if they fully represent all details. Reports of implicit change detection in the absence of awareness are consistent with the notion that observers' representations are more complete than previously thought. However, to provide convincing evidence, studies must separate implicit detection from explicit processes. This article reexamines the 3 primary claims of implicit change detection and, after replicating original findings, provides theoretical and empirical support for alternative, explicit explanations. Even if observers do represent more of the scene than previously thought, change detection might occur only through explicit comparisons.