Emociones han sido un objeto de estudio importante en Psicologia. Otro tema bien estudiado es la percepcion social. Esta tesis estudia como el de los estimulos afectivos puede modificar la respuesta emocional provocada por dichos estimulos. El objetivo principal de la tesis era investigar esta cuestion mediante diferentes metodos. Hemos empleado fotografias clasificandolas atendiendo a dos dimensiones afectivas basicas - valencia afectiva y activacion. Para operativizar la variable contenido dividimos los estimulos sociales en tres categorias (sin personas, con una persona y con dos o mas personas) y introducimos una escala de autoinforme, social, que participantes usaron para evaluar el grado de interaccion en imagenes. La parte experimental consiste en cuatro estudios. En el Experimento 1 obtuvimos la evaluacion subjetiva de 200 imagenes afectivas para disponer de una base de imagenes equiparadas en valencia afectiva, activacion y interaccion para usarlas en sucesivos experimentos. Ademas, estudiamos las relaciones entre las mencionadas dimensiones. En el Experimento 2 estudiamos respuestas perifericas: actividad electrodermica, actividad electromiografica facial y respuesta de parpadeo reflejo de sobresalto durante la observacion pasiva de imagenes. En el Experimento 3 investigamos la actividad cerebral mediante la electroencefalografia durante una tarea atencional con imagenes como distractores. En el Experimento 4 introducimos una tecnica novedosa para la captacion de los movimientos de cabeza como indicadores del estado emocional. Segun la evaluacion subjetiva (Experimento 1) las imagenes agradables se valoraron como las mas sociales. Observamos una correlacion positiva entre la escala de interaccion y las escalas de valencia afectiva y activacion solo en el caso de las imagenes agradables: cuanto mas se percibe una imagen agradable, tanto mas agradable y activadora es. La actividad electromiografica del musculo corrugador superciliar era mayor en respuesta a imagenes desagradables con una persona que a imagenes desagradables sin personas (Experimento 2). La actividad electrodermica era mayor cuando participantes vieron las imagenes desagradables con una persona que cuando vieron las imagenes desagradables sin personas y con dos o mas personas. Pensamos que la situacion a uno provoco cierto estado de incertidumbre. Para explicar este resultado recurrimos al efecto del espectador, que consiste en que los espectadores de situaciones de emergencia son mas responsables. Este efecto parece manifestarse en los patrones de la actividad periferica registrados por nosotros. La modulacion emocional de la respuesta de parpadeo no fue influida por el social. Probablemente, la base cerebral de percepcion es difusa y no puede modular directamente una respuesta tan rapida. En la tarea atencional con imagenes afectivas como estimulos distractores, las imagenes con dos o mas personas provocaron una respuesta mas rapida (Experimento 3), lo que interpretamos a la luz de la teoria de la facilitacion que postula que los humanos desempenan unas tareas mejor en presencia de otras personas gracias a un aumento de activacion. Este efecto conductual iba acompanado de la modulacion de los potenciales evocados electroencefalograficos. La latencia de las ondas N1a, P2a, P2p, N2a y N2p era mas corta y la amplitud de las ondas N2a y P3 era mayor en respuesta a imagenes con una y dos o mas personas. Es decir, el procesamiento de tales estimulos es mas rapido, pero requiere mas recursos. La dispersion espacial de los movimientos de cabeza era menor durante la vision de imagenes agradables con una y dos o mas personas que durante la vision de imagenes agradables sin personas (Experimento 4). Este hallazgo esta de acuerdo con una vision evolucionista que supone que el ambiente agradable provoca una sensacion de seguridad que puede provenir de la naturaleza de humanos. Emotions have always been an important subject of study in Psychology. Another well-studied topic is perception. This thesis studies how the content of affective stimuli can modulate different responses provoked by such stimuli. The main aim of the present work was to investigate this question using different methods. We employed photographs and classified them according to two main affective dimensions - affective valence and arousal. To operationalise the term social content, first, we divided stimuli into three categories (without people, with one person, and with two or more people) and, second, we introduced a self-report scale, social that participants used to evaluate the degree of interaction depicted in pictures. The experimental part consists of four studies. In Experiment 1 we obtained the subjective ratings of 200 pictures to build a set of pictures balanced in affective valence, arousal level and interaction, in order to use these pictures in the following experiments. We also investigated the relationships between the aforementioned dimensions. In Experiment 2 we measured peripheral variables: electrodermal activity, facial electromyography and modulation of the startle blink response during the passive observation of pictures. In Experiment 3 we investigated the brain activity using the evoked potentials of the electroencephalogram during an attentional task with pictures as background distracters. In Experiment 4 we introduced a new technique for capturing the head movements as an indicator of emotional state and predisposition to act. According to subjective evaluation (Experiment 1), pleasant pictures were evaluated as the most ones. We observed a positive correlation between the interaction ratings and the affective valence and arousal ratings only for pleasant pictures (but not for neutral and unpleasant pictures): the more a pleasant picture was, the more pleasant and arousing was considered. The electromyographic activity of the corrugator supercilii muscle was greater to unpleasant pictures with one person than to unpleasant pictures without people (Experiment 2). Electrodermal response was greater when participants watched unpleasant pictures with one person than when they watched unpleasant pictures without people and with two or more people. We think that the one-to-one situation caused a state of uncertainty, and for a possible explanation we referred to the bystander effect that consists in that in emergency situations observers are more responsible when they are alone. This effect seems to manifest itself in the physiological patterns we obtained. However, the startle blink response was not influenced by the content. A possible interpretation is that the brain underpinnings of perception are diffuse and cannot directly modulate the startle blink reflex that develops very quickly. In the attentional task pictures with two or more people provoked a faster response (Experiment 3). We interpret this result in the light of the theory of facilitation that postulates that humans perform a task better in the presence of others thanks to the increase of arousal. The data on the cerebral activity, as measured by event-related potentials, revealed that the latency of N1a, P2a, P2p, N2a and N2p was shorter and the amplitude of the N2a and P3 was larger to pictures with one person and two or more people. Thus, the processing of stimuli is faster but requires more resources. The spatial spread of head movement was lesser when viewing pleasant pictures with one and two or more people (Experiment 4). This finding is consistent with an evolutionary theory that assumes that pleasant environment causes a sense of security that might come from the nature of humans that allowed them to survive.