Robust selectivity to two-object images in human visual cortex

doi:10.1016/j.cub.2010.03.050

Curr Biol. Author manuscript; available in PMC 2011 May 11.

Published in final edited form as:

Curr Biol. 2010 May 11; 20(9): 872–879.

Published online 2010 April 22. doi: 10.1016/j.cub.2010.03.050

PMCID: PMC2869389

NIHMSID: NIHMS193485

Robust selectivity to two-object images in human visual cortex

Yigal Agam,¹ Hesheng Liu,⁴ Alexander Papanastassiou,⁶ Calin Buia,¹ Alexandra J. Golby,⁵ Joseph R. Madsen,⁶ and Gabriel Kreiman^1,^2,³^*

¹ Department of Ophthalmology and Kirby Neurobiology Center, Children’s Hospital, Harvard Medical School

² Center for Brain Science, Harvard University

³ Swartz Center for Theoretical Neuroscience, Harvard University

⁴ Massachusetts General Hospital

⁵ Department of Neurosurgery, Brigham and Women’s Hospital

⁶ Department of Neurosurgery, Children’s Hospital, Harvard Medical School

^* To whom correspondence should be addressed: Email: gabriel.kreiman/at/tch.harvard.edu

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SUMMARY

We can recognize objects in a fraction of a second in spite of the presence of other objects [1–3]. The responses in macaque areas V4 and inferior temporal cortex [4–15] to a neuron’s preferred stimuli are typically suppressed by the addition of a second object within the receptive field (see however [16, 17]). How can this suppression be reconciled with rapid visual recognition in complex scenes? One option is that certain “special categories” are unaffected by other objects [18] but this leaves the problem unsolved for other categories. Another possibility is that serial attentional shifts help ameliorate the problem of distractor objects [19–21]. Yet, psychophysical studies [1–3], scalp recordings [1] and neurophysiological recordings [14, 16, 22–24], suggest that the initial sweep of visual processing contains a significant amount of information. We recorded intracranial field potentials in human visual cortex during presentation of flashes of two-object images. Visual selectivity from temporal cortex during the initial ~200 ms was largely robust to the presence of other objects. We could train linear decoders on the responses to isolated objects and decode information in two-object images. These observations are compatible with parallel, hierarchical and feed-forward theories of rapid visual recognition [25] and may provide a neural substrate to begin to unravel rapid recognition in natural scenes.

RESULTS

We recorded intracranial field potentials (IFPs) from 672 electrodes (296 in different parts of visual cortex) in 9 subjects implanted with subdural intracranial electrodes. Subjects were presented with contrast-normalized grayscale images (100 ms duration) containing one or two objects. The single-electrode analyses focus on 24 visually selective electrodes.

Spatial summation

Figure 1 illustrates the IFP signals from a visually selective electrode in the left fusiform gyrus. Consistently with previous studies (e.g. [24, 26]), this electrode showed an enhanced response to human faces compared to other categories (Figure 1A). The activity elicited by two objects from the same category was almost indistinguishable from the activity in one-object images (Figure 1A). There was only a small attenuation when the preferred category was paired with a non-preferred category (Figure 1B). This robustness was largely independent of the non-preferred category (Figure S1A). We defined the IFP “response magnitude” as the signal range, R=max(IFP)−min(IFP), in the [50;300] ms interval after stimulus onset (Figure 1F). Because lack of visual selectivity could be confused with robustness [12], the single-electrode analyses were restricted to 24 electrodes that showed selectivity in one-object images (Experimental Procedures; see Figure S1B–E for more examples and Figure 2A for a normalized average plot). Some electrodes (e.g. Figure 1, S1C–D) showed two peaks in the responses to one-object or two-object images (also Figure 2A).

Figure 1

Example of robustness to the presence of a second object in intracranial field potential (IFP) recordings from human visual cortex

Figure 2

Comparison of the responses across positions and number of objects in the image

Object positions were randomized. If robustness to the second object were due to a small IFP receptive field surrounding only one position, we would expect to observe a bimodal response distribution. We did not observe any evidence for such bimodal distributions (Figure 1F, S1B4–E4). Furthermore, the IFPs to the preferred category were consistently stronger across different positions (Figure 1C–D). The position tolerance index (defined in Figure 2B) was 0.09±0.08 for single objects and 0.08±0.07 for two-object images (mean±SD), indicating only a modest response drop across positions. There was no clear preference for the top or bottom positions (Figure 2C). There was a weak correlation between the position tolerance index and the suppression index defined below (Pearson correlation coefficient (ρ)=0.26; p>0.05). These observations suggest that robustness to two-object images cannot be ascribed to a small IFP receptive field surrounding only the preferred category. However, the IFP response in both positions still allows for pooling over neurons with smaller receptive fields.

Single neuron responses in macaque area V4 [4, 5] or inferior temporal cortex [6–15] to two-object images are significantly attenuated in the presence of non-preferred objects within the receptive field (see however [16, 17]). To quantify the degree of suppression, let R₁ (R₂) indicate the response to category 1 (category 2) alone and R₁₂ indicate the response to both categories. There was a correlation between R₁₂ and max(R₁,R₂) even though R₁₂ was consistently below max(R₁,R₂) (Figure 1E, S2A). R₁₂ was also correlated with max(R₁,R₂) at the population level (Figure 2D, S2B). When considering individual exemplars, the mean suppression index (SI, defined in Figure 2E) was −0.09±0.16 (24 electrodes, n=80 exemplar pairs). When considering categories, the mean SI was −0.02±0.09 (24 electrodes, n=140 category pairs). We did not observe differences in SI between those electrodes that preferred human faces versus other categories (Figure 2D–E; S2B–C). The SI values were also similar in the [50;200] ms interval and in the target-present trials (Figure S2C). We considered several typical models for estimating the response to object pairs from the responses to the individual objects: maximum, average, unscaled power, scaled linear, normalization, scaled power and generalized linear (Figure S2D–I). The best fits were obtained using a two-parameter model: αmax(R₁, R₂) + βmin(R₁, R₂) (Figure S2F–H; see also [4, 27]). There was a stronger contribution from the first term (max) compared to the second term (min) (<α>=0.74±0.18, <β>=0.13±0.28, mean±SD, Figure S2G).

Single-trial decoding in two-object images

We asked whether we could decode visual information in single presentations of two-object images from the activity of individual electrodes or electrode ensembles using a machine-learning approach [23, 24, 28]. Figure S3A–D illustrates single-trial responses from the electrode in Figure 1. We extracted three parameters from the single-trial responses ([50;300] ms): the minimum voltage time, the maximum voltage time and the response magnitude (Figure S3). We used the responses to one-object images to train a binary support vector machine classifier (SVM) with a linear kernel to indicate the presence or absence of the preferred category. The classification performance (CP) was evaluated using the responses to two-object images (CP=50% indicates chance levels whereas CP=100% indicates perfect performance; see also Figure S5F). In Figure S3, the classification performance was 71±1% (mean±SD). To assess the statistical significance of the CP values, we computed the distribution of CP values in 100 iterations where we randomly shuffled the object labels. The CP values ranged from 51% to 77% (60±7%; mean±SD, n=24 electrodes; Figure 3A). Of the 24 visually selective electrodes (based on single-object images), 21 electrodes (88%) showed a significant CP in the two-object condition (training on single objects and testing on two-object images; _singleCP_2-object).

Figure 3

Decoding visual information in single presentations of two-object images

We previously examined single-trial responses to one-object images [23, 24] by training a classifier with a fraction of the repetitions (70%) and evaluating CP with the remaining repetitions (“CP_selectivity”). CP_selectivity was correlated with _singleCP_2-object (Figure S4B). The points in Figure S4B were below the diagonal, indicating a drop in CP when extrapolating from one-object images to two-object images. Of the 18 locations with >10 electrodes (Table S1), four locations yielded significant CP in two-object images: the inferior occipital cortex, the lateral fusiform gyrus, the parahippocampal gyrus and the inferior temporal cortex [29] (Figure S4C–D). Yet, we note that our sampling is far from exhaustive.

We extended the machine learning approach by considering a “pseudopopulation” defined by combining electrodes across the entire data set (e.g. [23, 24]). We concatenated the responses from multiple electrodes (we did not consider interactions among electrodes). Electrode selection for the pseudopopulation was based on selectivity to one-object images (Figure 3B) or electrode location (Figure 3D–G). The performance of a pseudopopulation consisting of 45 electrodes is shown in Figure 3B. The ensemble of electrodes yielded a stronger extrapolation to two-object images than the individual electrodes (cf. Figure 3A vs. 3B). The main locations that yielded significant classification performance were the inferior occipital gyrus, the lateral fusiform gyrus and inferior temporal cortex (Figure 3D–G).

We also examined the performance of the classifier when it was trained using the responses to two-object images. In Figure S5B–F we compare different ways of training and testing the classifier’s performance. Overall, the CP values for these different variations were similar (Figure 3C and S5). These results suggest that an algorithm that learns to recognize object categories from the neural signals in human temporal cortex can be trained in the presence or in the absence of another object in the image.

Temporal dynamics and latencies

Physiological signals in the human temporal cortex show a latency of ~100–150 ms (e.g. [24]; see also similar latencies in macaques [22, 23, 30] and human scalp signals [1, 31]). The selectivity in the IFPs in two-object images was apparent from the beginning of the evoked IFP signal (Figure 1B, S1B–E). We computed the latency of the responses to two-object images (Figure 4A). If the selective responses were due to attentional shifts or fast saccades to one of the objects, we might expect that the responses to two-object images would show a longer latency compared to one-object images [32–34]. In contrast, we did not observe significant differences between the response latencies to one-object images (167±45 ms, mean±SD, n=24) and two-object images (157±37 ms, mean±SD, n=24) (Figure 4A; two-tailed t-test p>0.3). Furthermore, there was a weak but significant correlation between the latencies to one-object and two-object images (ρ=0.67; Figure 4B). To further examine the temporal evolution of the IFP responses, we computed the suppression index (SI) as a function of time in 25 ms bins. Overall, SI remained close to 0 in the [50;300] ms interval (e.g. compare thin and thick traces in Figure S1B3–E3) and there were no consistent monotonic changes in SI over time (Figure 4C).

Figure 4

Response latencies and temporal evolution of the response suppression in two-object images

DISCUSSION

We can rapidly recognize objects within 100–200 ms of seeing a complex scene [1, 2, 25]. Object recognition in multi-object images poses a challenging problem due to difficulties in segmentation, increased processing time and response attenuation [4–15]. Given these challenges, what are the neural mechanisms that underlie rapid recognition in multi-object images? Attention may help filter out “irrelevant” information enhancing certain locations/features/objects. While attention plays an important role in crowded images [19–21], it remains difficult to explain the high performance during brief presentation of a novel image by serial attentional shifts [1–3, 14, 25]. Alternatively, the first sweep of information through the ventral stream may contain sufficient information to account for recognition in multi-object images. We evaluated this possibility by quantifying how well we can decode information from IFP recordings in human visual cortex in response to two-object images. We report that the representation in inferior occipital gyrus, fusiform gyrus and inferior temporal cortex can support object recognition even in the presence of a second object in the image. The rapid onset of the selective responses suggests that recognition in two-object images may not require additional computational steps.

The degree of response suppression reported here is lower than in previous studies [4–15] (see however [16, 17, 35]). Several non-exclusive factors may account for these differences including the species (macaques versus humans), brain areas (it remains difficult to establish one-to-one homologies between macaques and humans), recording techniques (field potentials versus action potentials), tasks and stimulus characteristics (particularly distance between objects and whether the two objects appear in the same hemifield [10, 13, 16]). The biophysical nature underlying the IFPs remains only poorly understood. IFPs may reflect synaptic potentials averaged over many neurons [36]. We speculate that the IFPs may provide a “population view” that shows enhanced robustness to two-object images compared to individual neurons.

A possible mechanism to account for robustness to two-object images would be rapid attentional shifts and/or saccades to the electrode’s preferred category. While this possibility cannot be entirely ruled out here, it seems to be an unlikely account of our observations. (i) Subjects could not predict where to saccade before image onset (positions were randomized). Additionally, the response distributions were unimodal (Figure S1) and behavioral performance was indistinguishable across positions. These observations render it unlikely that the results could be accounted by pre-onset fixation or spatial attention to one location. (ii) Adding saccade times of 200–300 ms [37, 38] and latencies of 100–150 ms [24] to the 100 ms stimulus flash, physiological responses elicited by saccades would take place after ~300 ms. (iii) In one subject where we monitored eye position, we did not observe any differences in the responses or suppression index (SI) that could be explained by eye movements. (iv) We observed similar SI when the analysis interval was restricted to [50;200] ms (Figure S2C). (v) Similar SIs were observed for electrodes that preferred faces or other categories (Figure 2D–E, S2B–C). Furthermore, in some cases, there were different electrodes in the same subject that preferred different categories; a category-specific attentional account would necessarily fail to explain the responses in some electrodes (e.g. Figure S1B–C). (vi) The SIs during the initial 300 ms were unaffected by target presence (Figure S2C). (vii) The latencies to one-object images and two-object images were similar (Figure 4). Taken together, observations (i)–(vii) do not rule out an attentional account of our findings but delimit the possible roles of attention. The physiological characterization of the spatial summation properties (Figures 2, ,3,3, S2), category preferences (Figures 3, S4, S5), task demands (Figure S2C) and timing (Figure 4) places strong constraints on how attentional shifts should be incorporated into biophysically-plausible computational circuits for visual recognition (e.g. [25]).

We used two relatively large objects surrounded by a gray background. Visual recognition becomes more challenging and reveals serial attentional shifts upon increasing the amount of “clutter” in the image. Therefore, we do not claim that the initial physiological signals in temporal cortex can account for visual recognition under all possible visual conditions. Our work does suggest, however, that the presence of two objects and modest response suppression do not completely disrupt visual recognition by the initial sweep of visually selective signals.

EXPERIMENTAL PROCEDURES

Subjects

Subjects were nine patients (10–47 years old, 6 right-handed, 3 males) with epilepsy admitted into either Children’s Hospital Boston (CHB) or Brigham and Women’s Hospital (BWH) to localize the seizure foci for potential surgical resection [39, 40]. The tests were approved by the IRBs at both Hospitals and were performed under the subjects’ written consent.

Recordings

Subjects were implanted with 64 to 88 intracranial subdural grid (64%) or strip (36%) electrodes (8 subjects) or intracortical depth electrodes (1 subject) as part of the surgical approach to treat epilepsy. The grid/strip electrodes were 2 mm in diameter, with 1 cm separation and impedances below 1kΩ (Ad-Tech, Racine, WI). The signal from each electrode was amplified (×2500), filtered between 0.1 and 100 Hz and sampled at 256 Hz at CHB (XLTEK, Oakville, ON) and 500 Hz at BWH (Bio-Logic, Knoxville, TN). A notch filter was applied at 60Hz to remove line noise artifacts (5-th order bandstop Butterworth filter between 58 and 62 Hz implemented in MATLAB’s butter function). We refer to the voltage signal as “intracranial field potential” (IFP). Subjects stayed in the hospital 6 to 9 days. The number and location of the electrodes were determined by clinical criteria (Table S1). In one subject, we monitored eye movements using a non-invasive system from ISCAN (DTL-300, Woburn, MA) which provides a spatial resolution of ~1 deg and a temporal scanning frequency of 60 Hz. We excluded from the analyses those electrodes that were considered to be part of the epileptogenic focus according to clinical criteria.

Stimulus presentation and task

Subjects were presented with grayscale images containing one or two objects. Objects belonged to one of five possible categories: animals, chairs, human faces, cars and houses. There were 5 exemplar objects per category and each exemplar object was contrast normalized. Images were presented for 100 ms, with a 1000 ms gray screen in between images. Images included one object (30%) or two objects (70%). The two objects were presented either above and below the fixation point (50%) or to the left and right of the fixation point (50%). In the one-object images, the object was randomly presented at one of the possible locations (above/below or left/right with respect to the fixation point) at the same size and eccentricity as in the two-object images. In the first three subjects, there were four possible positions (above, below, right, left of the fixation point). In the remaining six subjects, the task was restricted to two positions (above/below) to increase the number of repetitions at each position. Objects subtended ~3.4 degrees of visual angle and were presented with their center ~3.8 degrees from the fixation point. Subjects were asked to fixate on the fixation point. Object order and positions were randomized. The duration of each session (and therefore the number of repetitions) depended on clinical constraints and subject fatigue (min duration = 6 mins, max duration = 29 mins, mean=14.8±8.0 mins). In many cases we ran several sessions per subject (min=1 session, max=4 sessions, mean=2.9±1.1 sessions). The first two presentations within each block were not considered for analyses to avoid potential non-stationary effects. Data from all sessions for a given subject were pooled together for analyses. On average, the total number of presentations was 1156±451; 338±131 one-object images (67±13 per category) and 650±242 two-category images (64±66 per category pair). At the onset of each block (50 images per block) a target category was announced by a written word presented on the screen. Subjects had to indicate by pressing designated “yes” and “no” keys whether or not each image included an object from the target category. The overall performance was 92±12% correct (range=75–100 % correct; one-object images 92±10%; two-object images: 91±13%). The average reaction time was 630±90 ms (one-object images: 625±103 ms; two-object images: 640±96 ms).

Data Analyses

Electrode localization

To localize the electrodes, we integrated anatomical information from preoperatively acquired MRI and spatial information of electrode positions provided by postoperatively acquired CT. For each subject, the 3-D brain surface was reconstructed and an automatic parcellation was performed using Freesurfer [24]. CT images were first registered to the MRI using a 3-D affine transform based on multiple fiducial marks. After the co-registration, electrodes were projected onto the nearest brain surface (Table S1). Electrodes were superimposed on the reconstructed brain surface for visualization purposes in the figures. Talairach coordinates and brain renderings for all 672 electrodes are available upon request.

IFP response definition

We focused on the initial part of the IFP response (50 to 300 ms after stimulus onset) because (i) it is more strongly correlated with the visual stimuli; (ii) it is less affected by potential effects of eye movements or attentional shifts [37]; (iii) we can directly compare the responses against the same intervals used in macaque studies (e.g. [15, 23]) and (iv) we can more readily compare the initial sweep of the response with feed-forward models of object recognition [25]. We have previously characterized IFP signals based on different response definitions [24]. We define the “IFP response magnitude”, R, as the voltage range (max(V)−min(V)) in the [50;300] ms time interval. An electrode was defined to show visual selectivity if a one-way ANOVA across object categories based on the IFP response magnitude to the one-object images yielded p<0.01 [24]. Visually selective electrodes responded to an average of 1.45 categories (ranging from 1 to 3 categories). Unless stated otherwise (Figure S2C), the analyses focus on those trials where the target category was absent to remove the possible influence of the target on the spatial summation properties. The initial IFP response magnitude was not significantly affected by the presence or absence of the target category (Figure S2C). Many electrodes did show significant differences between target and non-target trials beyond 300 ms after stimulus onset. However, the physiological responses beyond 300 ms are beyond the scope of the manuscript.

Spatial summation properties

We compared the responses to one-object images against two-object images (e.g. Figure 2 and S2). We also considered several biophysically-plausible simple models [4] to explain the response to two-object images based on the responses to the constituent single objects (Figure S2). When fitting these models, we used the function nlinfit in MATLAB.

Classifier analysis

We used a machine-learning approach [23, 28] to read out visual information from the IFP responses in single trials. We considered the [50;300] ms interval and we defined three features of the IFP signal: the minimum voltage time (t_min), the maximum voltage time (t_max) and the response magnitude R (Figure S3A–D). For each electrode i, we constructed a response vector: [ equation M1

, Rⁱ]. Several other ways of defining the response vector for each electrode were described previously [24]. This response vector is defined for each individual trial (there is no averaging of responses across trials). The classifier approach allows us to consider each electrode independently or to examine the encoding of information by an ensemble of multiple electrodes. When considering a set of N electrodes, we assumed independence across electrodes and concatenated the responses to build the ensemble response vector: [ equation M2

, R¹,…,

, R^N]. The results shown throughout the manuscript correspond to binary classification between a given category and the other categories (see Figure S5F for multiclass classification). In a binary classifier, chance corresponds to 50% (horizontal dashed line in the plots). We used a support vector machine (SVM) classifier with a linear kernel to learn the map between the ensemble response vectors and the object categories. In all cases, the data were divided into two non-overlapping sets, a training set and a test set. We examined different ways of separating the data into a training set and a test set (Figure S5). Throughout the text, we report the proportion of test repetitions correctly labeled as “Classification performance” (CP). To assess the statistical significance of the classification performance values, we compared the results against those obtained after performing 100 iterations where we randomly shuffled the object labels. We considered CP to be significant if performance was more than 3 standard deviations above the null hypothesis.

Latency

We used two different definitions to compute the response latency as described in Figure 4.

Supplementary Material

Click here to view.^{(3.2M, pdf)}

Acknowledgments

We would like to thank the patients for their cooperation. We also thank Nuo Li, David Cox, Geoffrey Ghose and Rufin Vogels for comments on the manuscript and Sheryl Manganaro and Paul Dionne for technical assistance. We acknowledge financial support from the Epilepsy Foundation, the Klingenstein Fund, the Whitehall Foundation, NIH grant 1R21EY019710 and an NIH New Innovator Award (1DP2OD006461).

Footnotes

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