def only_extreme_entropy(df):
    """
    Returns the trials that involve words with extreme values of entropy (either positive or negative).
    :param df: dataframe with responses.
    :return: dataframe only with responses of trials with extreme values of entropy.
    """
    extremes=["extreme_neg","extreme_pos"]
    df = df.loc[df["condition"].isin(extremes)]
    return df